GMAT Problem Solving Discussions

Hey Sri,
yes, when m = 4k then n = 4k+2 but reverse is true as well
i.e when m = 4k + 2 , then n = 4k

Likewise, when m= 4k+1 then n= 4k+3 and reverse is true as well
i.e when m=4k+3 then n=4k+1

Hence,
(25C1*25C1) + (25C1*25C1) + (25C1*25C1) + (25C1*25C1) /100*100 = 1/4


I thought of it this way:

7^k ends in 7,9,3,1
Probability : (7^m + 7^n) is div by 5

a = 7^m
b = 7^n

a+b is div by 5 only in 2 cases = 7+3, 9+1 (units digit)

1) 7+3: p(a ending in 7) = 1/4, p(b ending in 3) = 1/4
2) 9+1: p(a ending in 9) = 1/4, p(b ending in 1) = 1/4

p(a+b=zero(mod)5) = (1/4 * 1/4 + 1/4 * 1/4 ) *2 = 1/4 (multiplied by 2 since combination can be 7+3 or 3+7. Similarly 9+1 or 1+9)

Ans 1/4.

PS: I had forgotten to multiply by 2 initially and hence ended up with 1/2. After seeing the correct ans, corrected my ans as well:o
Hey Sri,
yes, when m = 4k then n = 4k+2 but reverse is true as well
i.e when m = 4k + 2 , then n = 4k

Likewise, when m= 4k+1 then n= 4k+3 and reverse is true as well
i.e when m=4k+3 then n=4k+1

Hence,
(25C1*25C1) + (25C1*25C1) + (25C1*25C1) + (25C1*25C1) /100*100 = 1/4



hey thanks bhavin...
infact i was waiting for u to take a stab at it ..

i ve been following ur posts.. there are so many things to learn from ur posts
thanks again ..
In how many ways can one choose 6 cards from a normal deck of cards so as to have all suits present?

a. (13^4) x 48 x 47
b. (13^4) x 27 x 47
c. 48C6
d. 13^4
e. (13^4) x 48C6

I am lost.. Got several methods and several different answers..
bhavin I think u r d one to rescue.. :

Possible answers:
1)13C1 x 13C1 x 13C1 x 13C1 x 48C2 (not in option)
2)13C1 x 13C1 x 13C1 x 13C1 x 48 x 47
3) 1 x 1 x 1 x 1 x 48C2


FYI...I saw this question in IMS material today. There is a typo in answer choice B. It needs to be 24 instead of 27. Which translates to 13^4 * 48C2 as the answer.

Hey Vikram,

FYI:I don't have IMS material.. :)
neways to update u even 13^4 * 48C2 is not d answer if at all IMS ppl have mentioned dat as d OA..

for the correct answer refer to bhavin's post..


Vikram2010 Says
FYI...I saw this question in IMS material today. There is a typo in answer choice B. It needs to be 24 instead of 27. Which translates to 13^4 * 48C2 as the answer.

If x is equal to the sum of even integers from 400 to 600, both inclusive, and y is the number of even integers 400 to 600, both inclusive, what is the value of x+y ?

A)50601
B)50600
C)56002
D)57002
E)50000

If x is equal to the sum of even integers from 400 to 600, both inclusive, and y is the number of even integers 400 to 600, both inclusive, what is the value of x+y ?

A)50601
B)50600
C)56002
D)57002
E)50000

Remember a simple formula:
when the end terms are inclusive{divided by 2 because of even integers, a more generalized form is {(n2-n1)/N}+1 where N is the dividing number)

no. of terms={(n2-n1)/2}+1

if non-inclusive
no. of terms={(n2-n1)/2}

here we have y={(600-400)/2}+1=101

Now Sum of even terms

Use a simple Sum of AP=n(a+l)/2 where a-first term and l-last term of the series.

thus x=101(400+600)/2=50500

x+y=50601 i.e. option A

This one is straight from OG 11.
------------

A certain Junior class has 1000 students, and a certain senor class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

A) 3/40000
B) 1/3600
C) 9/2000
D) 1/60
E) 1/15

This one is straight from OG 11.
------------

A certain Junior class has 1000 students, and a certain senor class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

A) 3/40000
B) 1/3600
C) 9/2000
D) 1/60
E) 1/15

60 sibling pair means 60 students in Junior Class and 60 students in senior class.

P(e)=(60C1/1000C1)*(60C1/800C1)=9/2000 i.e. option C

Pls confirm the OA.
60 sibling pair means 60 students in Junior Class and 60 students in senior class.

P(e)=(60C1/1000C1)*(60C1/800C1)=9/2000 i.e. option C

Pls confirm the OA.


This is exactly what I had. OG 11 has it wrong looks like. As per the book, the OA is: A

The explanation is 60/1000 for juniors and 1/800 for seniors so multiply both for A.

You and I had 60/1000 * 60/800
Which I think is correct.

By the way, you are working out my problems, and I am correcting your SCs in the other thread...totally cool timing. Don't we love PG forum !! :thumbsup:
If x is equal to the sum of even integers from 400 to 600, both inclusive, and y is the number of even integers 400 to 600, both inclusive, what is the value of x+y ?

A)50601
B)50600
C)56002
D)57002
E)50000


sum of even numbers-n^2/4+n/2
applying this the answer is A.
60 sibling pair means 60 students in Junior Class and 60 students in senior class.

P(e)=(60C1/1000C1)*(60C1/800C1)=9/2000 i.e. option C

Pls confirm the OA.


hey Vyom πŸ˜ƒ hwdy ..!!!

Let me explain it in this way ...

let Sibling Set --> all people who have a sibling --> all 120 people

(60C1/1000C1)*(60C1/800C1) --> probability of selecting two people from the Sibling Set (one from each class)and not Siblings

Let (A ,B are Siblings) (C,D are siblings)...

U found the probabilty of picking Aand C from both the classes and not A and B ..!!


Here goes my explanation ...!!

Pick 1 guy from a class of 1000 who belongs to the sibling set

So it s 1 * (60C1/1000C1) ....

Pick one guy from the class of 800 who belongs to he sibling set

Then it is (60C1/800C1) but

these two just belong to the sibling set but not neccesarily siblings
Picking one guy from 60 from a the above set will assure you that the guy is the other sibling from the 1000 class
=(1/60 )*(60C1/800C1)


So final probability
= 1 * (60C1/1000C1) *( 1/60 )*(60C1/800C1)

= 60*60/(60*1000*800)

= 3/40000


P.S: pathetic explanation.....please excuse
P.S.S: correct me if i am wrong
hey Vyom πŸ˜ƒ hwdy ..!!!

Let me explain it in this way ...

let Sibling Set --> all people who have a sibling --> all 120 people

(60C1/1000C1)*(60C1/800C1) --> probability of selecting two people from the Sibling Set (one from each class)and not Siblings

Let (A ,B are Siblings) (C,D are siblings)...

U found the probabilty of picking Aand C from both the classes and not A and B ..!!


Here goes my explanation ...!!

Pick 1 guy from a class of 1000 who belongs to the sibling set

So it s 1 * (60C1/1000C1) ....

Pick one guy from the class of 800 who belongs to he sibling set

Then it is (60C1/800C1) but

these two just belong to the sibling set but not neccesarily siblings
Picking one guy from 60 from a the above set will assure you that the guy is the other sibling from the 1000 class
=(1/60 )*(60C1/800C1)


So final probability
= 1 * (60C1/1000C1) *( 1/60 )*(60C1/800C1)

= 60*60/(60*1000*800)

= 3/40000


P.S: pathetic explanation.....please excuse
P.S.S: correct me if i am wrong


Yep. I believe.. I didnt focussed myself on this part of the problem..
Newayz..thanks for highlighting it..
This is exactly what I had. OG 11 has it wrong looks like. As per the book, the OA is: A

The explanation is 60/1000 for juniors and 1/800 for seniors so multiply both for A.

You and I had 60/1000 * 60/800
Which I think is correct.

By the way, you are working out my problems, and I am correcting your SCs in the other thread...totally cool timing. Don't we love PG forum !! :thumbsup:

Yaah sure Vikram.... We do LOVE PG :thumbsup: ...PG Rawks

60 pairs = 120
60 in Junior and 60 in Senior class..

Junior class has 1000 students
Senior class has 800 students

Probab of selecting on of the 60 students who have a twin in senior class is; (60/1000) = P(A)

Probab of finding his match is; (1/800) = P(B)

P(A)xP(B) = (60/1000)x(1/800) = (60/800,000) = (6/80,000) = (3/40,000)

answer is (3/40,000)... that is option A

This one is straight from OG 11.
------------

A certain Junior class has 1000 students, and a certain senor class has 800 students. Among these students, there are 60 sibling pairs, each consisting of 1 junior and 1 senior. If 1 student is to be selected at random from each class, what is the probability that the 2 students selected will be a sibling pair?

A) 3/40000
B) 1/3600
C) 9/2000
D) 1/60
E) 1/15

60 sibling pair means 60 students in Junior Class and 60 students in senior class.

P(e)=(60C1/1000C1)*(60C1/800C1)=9/2000 i.e. option C

Pls confirm the OA.

This is exactly what I had. OG 11 has it wrong looks like. As per the book, the OA is: A

The explanation is 60/1000 for juniors and 1/800 for seniors so multiply both for A.

You and I had 60/1000 * 60/800
Which I think is correct.

By the way, you are working out my problems, and I am correcting your SCs in the other thread...totally cool timing. Don't we love PG forum !! :thumbsup:

Puys pls help me to solve this question

Ken left a job paying $75k per year to accept a sales job paying $45k per year plus 15 percent commission.If each of his sales is for $750,what is the least number of sales he must make per year if he is not to lose money because of the change?
A-40
B-200
C-266
D-267
E-600

Puys pls help me to solve this question

Ken left a job paying $75k per year to accept a sales job paying $45k per year plus 15 percent commission.If each of his sales is for $750,what is the least number of sales he must make per year if he is not to lose money because of the change?
A-40
B-200
C-266
D-267
E-600


15% (750 x n) = 75000 - 45000
n=266.666
This is for the bear minimum number of sales. 266 will not be sufficient. Therefore 267.

IMO: D

Puys, please help...

a juice manufacturer organized taste - testing sessions featuring four brands of orange juice, A, B , C and D. All customers who participated told the organizer which variety they they thought was the best. exactly 61% preerred brand A and exactly half as many prefereed brand B. Only 65 chose brand C. which of the foll could be the number of customers who liked brand D?
a> 8
b> 11
c>14
d>20
e>31

Puys, some more...

1. Heights of women in a city follows a normal distribution with mean 160 cm and standard deviation of 6 cm. In a normal distribution, only 0.0063% of the population is not within 4 standard devistions of the mean. If 5 women are more than 184 cm tall, approximately how many women stay in the city?
a>16000
b>40000
c>80000
d> 100,000
e> 160,000


2. How many positive integers less than 100 have exactly 4 odd factors and no even factors?

a>13
b>14
c>15
d>16
e>17

Puys, please explain ur methods
the OA are as follws:
1.E
2.C

Puys, please help...

a juice manufacturer organized taste - testing sessions featuring four brands of orange juice, A, B , C and D. All customers who participated told the organizer which variety they they thought was the best. exactly 61% preerred brand A and exactly half as many prefereed brand B. Only 65 chose brand C. which of the foll could be the number of customers who liked brand D?
a> 8
b> 11
c>14
d>20
e>31


Brand A-61%
Brand B-30.5%
Brand C & D- 8.5%

C is 65 so 8.5% = 65+d

only when d = 20 100% is integer i.e if d=20 than 8.5% is 85 and 100% is (100/8.5)*85 = 1000

Ur right.. OA is option D