Official Quant thread for CAT 2013

@Abir1103 said:
@chillfactor sir the answer 1.5 is correct......bt i'm unable to understand the soln. can u plz explain it
Expected weight
= Summation of all {(weight gain)*(probability of gaining that much weight)}

Now, following cases needs to be considered
-> he picked first capsule, probability is 1/3 and weight gained will be 1 kg

-> he picked second capsule, probability is 1/3 and weight gained will be 0 kg

-> he picked third capsule, prob is 1/3 and weight gain is 2 kg. But he has to pick one more capsule also, so 1/3 probability for first and second each and weight gain will be 1 and 0 respectively. So, overall 1/9 probability of gaining 3kg and 1/9 for 2kg.

-> he picked third capsule, then again he picked third capsule, so, now probability of gaining 5 kg is 1/27 (if he picks first capsule) and probability of gaining 4 kg will be 1/27 (if he picks second capsule)

and so on....

add all these cases and get the expected weight gain as 1.5

65% of students in a class have taken subject A, 80% B, 75% C & 85% D. At least what percentage of students have taken alll four subjects??

Guys help me out with gud explanation

@fire123 said:
65% of students in a class have taken subject A, 80% B, 75% C & 85% D. At least what percentage of students have taken alll four subjects??Guys help me out with gud explanation
is it 5% ?
@fire123 said:

65% of students in a class have taken subject A, 80% B, 75% C & 85% D. At least what percentage of students have taken alll four subjects??

Guys help me out with gud explanation

If you add all the percentages,it would result in 305% ,this means that if all the four subjects are to be taken by particular student,then we have to subtract the percentage of students who have taken 3 subjects at most.
Thus,it would come as 305-300=5%
@RDN said:
Consider values modulo 9, then take all 2,3,4,8 and leave out all 1,5,6,7 and finally take one mod 0. Total =82

I am not getting the approach...can you please elaborate that why you have taken all 2,3,4,8 and left 1,5,6,7..???
@fire123 said:

65% of students in a class have taken subject A, 80% B, 75% C & 85% D. At least what percentage of students have taken alll four subjects??

Guys help me out with gud explanation

I+II+III+IV = 100 --(1)
I+2II+3III+4IV = 305 --(2)
(2)-(1)
II+2III+3IV = 205
Try to maximize II and III;
(make sure that the sum of II,III and IV doesn't exceed 100)
II=0
III=95
IV=5
Hence 5% :)
Out of 200 in an aquarium, 99% are red. How many red fish must be removed in order to reduce the percentage of red fish to 98%?
@jain4444 said:
Out of 200 in an aquarium, 99% are red. How many red fish must be removed in order to reduce the percentage of red fish to 98%?
100
@ChirpiBird said:
A point P inside an equilateral triangle, ABC is located at a distance of 3,4 and 5 from A, B , C respectively. What is the area of the triangle.@iLoveTorres try karo!!
Please find my solution in the attached pic. Area is 9+25 (root3)/4 sq. units

Alternatively there is a direct relation between the distances from vertex and the side length of an equilateral triangle given by 3(x^4+y^4+z^4+a^4)=(x^2+y^2+z^2+a^2)^2. So you may also plug in the values to find a^2 and hence the area of thr triangle.
@Narci said:
100
You are right :)

Most puys know multiplication tables by heart.

Would you puys like to share how to by-heart multiplication tables (from 12 to 30), permanently & fast?

Now it's time to flex some brain cells on divisibility basics.


Investigate all seven-digit numbers that have only the digits 5 and 7 and that are divisible by 35.
@bodhi_vriksha said:
You are right
how is it 100 pls explain
@bodhi_vriksha said:
You are right
how is it 100 pls explain
@iLoveTorres said:
how is it 100 pls explain
(198-n)/(200-n)=98/100
@iLoveTorres said:
how is it 100 pls explain
(198-x)/(200-x) = 98/100 =>x=100
@chillfactor said:
Expected weight= Summation of all {(weight gain)*(probability of gaining that much weight)}Now, following cases needs to be considered-> he picked first capsule, probability is 1/3 and weight gained will be 1 kg-> he picked second capsule, probability is 1/3 and weight gained will be 0 kg-> he picked third capsule, prob is 1/3 and weight gain is 2 kg. But he has to pick one more capsule also, so 1/3 probability for first and second each and weight gain will be 1 and 0 respectively. So, overall 1/9 probability of gaining 3kg and 1/9 for 2kg.-> he picked third capsule, then again he picked third capsule, so, now probability of gaining 5 kg is 1/27 (if he picks first capsule) and probability of gaining 4 kg will be 1/27 (if he picks second capsule)and so on....add all these cases and get the expected weight gain as 1.5

I did this in a shorter way, but couldn't prove I was not making a mistake 😞 Fell asleep over it...

Bascially I said, let the required answer be x.

Now x consists of 3 cases, each with 1/3rd prob.
Case 1 he gains 0
Case 2 he gains 1
Case 3 he gains 2 and starts the whole proces again so overall expectation which means an additional x. Hence here we can say he gains (2+x)
So adding it up x = 1/3 [0 + 1 + (2+x)] = 1/3 (x+3) which gives x = 1.5

Is there any flaw in this sir?

@Abir1103

regards
scrabbler

@saurabhlumarrai said:
I am not getting the approach...can you please elaborate that why you have taken all 2,3,4,8 and left 1,5,6,7..???
It need not be this specific combo. Basically
1+8 = 9
2+7 = 9
3+6 = 9
4+5 = 9
So if we take 1 out of the 2 in each set above it is fine. 2, 4, 6, 8 will work, but so will 1, 2, 3, 4 or 1, 3, 5, 7, or even 1, 7, 6, 4...up to you. Basically I cannot have both types in a set present together as then we would get a total divisible by 9.

regards
scrabbler

@ChirpiBird said:
A point P inside an equilateral triangle, ABC is located at a distance of 3,4 and 5 from A, B , C respectively. What is the area of the triangle.@iLoveTorres try karo!!
approx 12 rt(3)

Common guys the following is a challenge problem.


Investigate all seven-digit numbers that have only the digits 5 and 7 and that are divisible by 35.