Official Quant thread for CAT 2013

@ani6 said:
Divide 3903 between amar and akbar such that amars share at the end of 7 yrs is equal to akbars share at the end of 9 yrs at 4 percent pa rate of compound interestAns is amar is 2028 and akbar is 1875
let amar's be x
and akbar's will b 3903-x
akbar's 9 yr's is equal to amar's 7 yrs share.
it's given (3903-x)[1+1/25]^9=x[1+1/25]^7

solving ..
(3903-x)[26*26/25*25] = x
3903*26*26=x+ x676/625
or x = 3*676
x= 2028

which was amar's share. and akbar's is 1875
@jain4444 said:
E(300) = 80 sum of all co-primes = 300*80/2 = 12000
your answer is coincidentally right but your logic is wrong
according to your logic, the answer of this question -
Sum of all positive integers less than 6 and relatively prime to 3 should be 3 but it actually is 12.
This is because n*E(2n) is not equal to 2n*E(n)
@ravi.theja said:
6??n^5 - 17n^3 + 16n = n^5 - 16n^3 -n^3 + 16n = n^3 ( n^2 - 16) - n (n^2 -16 )(n^3 -n) (n^2 - 16 ) = n (n+1)(n-1) (n+4)(n-4) = [(n-1)*n * (n+1) ] is always divisible by 6. ;No..perfect rule for (n-4) (n+4 ) so it has to be 6?/
Ravi, i think The highest number that always divides n(n^2-1) is 24. (if n is odd.)
and it is 6 if n is even. Since n>=4 which will include odd values too, it should be 24.

@naveenkrs said:
your answer is coincidentally right but your logic is wrongaccording to your logic, the answer of this question -Sum of all positive integers less than 6 and relatively prime to 3 should be 3 but it actually is 12.This is because n*E(2n) is not equal to 2n*E(n)
sir prime factors of 3 = 3 and prime factors of 6 = 2 and 3 we cannot apply it here

but there prime factors of 150 = 2 , 3 and 5 and prime factors of 300 = 2 , 3 and 5
prime factors of both the numbers are same so we can apply this
@ChirpiBird said:
Ravi, i think The highest number that always divides n(n^2-1) is 24. (if n is odd.) and it is 6 if n is even. Since n>=4 which will include odd values too, it should be 24.
ya..gt it..ll edit my soltn :)
@ChirpiBird thanks !!!was going wrong in calculation.
@jain4444 said:
sir prime factors of 3 = 3 and prime factors of 6 = 2 and 3 we cannot apply it here but there prime factors of 150 = 2 , 3 and 5 and prime factors of 300 = 2 , 3 and 5 prime factors of both the numbers are same so we can apply this
your answer will never match if n is odd.
try this question - Sum of all positive integers less than 2250 and relatively prime to 1125.
The 150 contestants of Miss India 2010 are given individual numbers from 1 to 150. Several rounds happen before the final winner is selected. The elimination in each round follows an interesting pattern. In the 1st round starting from first contestant, every 3rd contestant is eliminated i.e. 1st, 4th, 7th, .... This repeats again from the first numbered (among the remaining) contestant in the next round (leaving 3, 5, 8, 9, ... ). This process is carried out repeatedly until there is only the winner left. What is the number of Miss India 2010?

(a) 93
(b) 48
(c) 119
(d) 38
(e) 140
@naveenkrs said:
The 150 contestants of Miss India 2010 are given individual numbers from 1 to 150. Several rounds happen before the final winner is selected. The elimination in each round follows an interesting pattern. In the 1st round starting from first contestant, every 3rd contestant is eliminated i.e. 1st, 4th, 7th, .... This repeats again from the first numbered (among the remaining) contestant in the next round (leaving 3, 5, 8, 9, ... ). This process is carried out repeatedly until there is only the winner left. What is the number of Miss India 2010?(a) 93 (b) 48 (c) 119 (d) 38 (e) 140
i have been trying this for the past 20 mins....got 140 by brute force....cld not find a pattern... :(
@naveenkrs said:
The 150 contestants of Miss India 2010 are given individual numbers from 1 to 150. Several rounds happen before the final winner is selected. The elimination in each round follows an interesting pattern. In the 1st round starting from first contestant, every 3rd contestant is eliminated i.e. 1st, 4th, 7th, .... This repeats again from the first numbered (among the remaining) contestant in the next round (leaving 3, 5, 8, 9, ... ). This process is carried out repeatedly until there is only the winner left. What is the number of Miss India 2010?(a) 93 (b) 48 (c) 119 (d) 38 (e) 140
150 in base 3 - 12120
left shift operator two times - 12012
12012 in base 10 = 2 + 3 + 54 + 81 ==140
@naveenkrs said:
your answer will never match if n is odd.try this question - Sum of all positive integers less than 2250 and relatively prime to 1125.
how to do this one?? i m clueless...the normal logic of N/2*E(N) won't work i guess...
@rkshtsurana said:
150 in base 3 - 12120left shift operator two times - 1201212012 in base 10 = 2 + 3 + 54 + 81 ==140
shift 2 times because there are 2 memebrs between any 2 eliminations??
i mean between 1 and 4 there are 2 numbers in the begining?? or any other concept? :lookaround:
@rkshtsurana said:
150 in base 3 - 12120left shift operator two times - 1201212012 in base 10 = 2 + 3 + 54 + 81 ==140
Wrong logic bro...I guess...

Suppose there were 157 contestants...
157 is 12211 in base 3...
shift operation makes it -> 21112....which is 203 in base 10, and it does not make sense...
so IMO, this method is either incorrect or has a certain limitation...
@rkshtsurana said:
150 in base 3 - 12120left shift operator two times - 1201212012 in base 10 = 2 + 3 + 54 + 81 ==140
Answer correct, logic wrong
@Logrhythm said:
how to do this one?? i m clueless...the normal logic of N/2*E(N) won't work i guess...
yes, it won't work.
We already know that sum of all positive integers less than n and relatively prime to n is nothing but n*E(n)/2.
We just need to calculate all the numbers between n and 2n which are relatively prime to n.

Let d1 n (where k =E(n)). Note that gcd(d, n) = 1 if and only if gcd(n+d, n) = 1.
so summation of d from n to 2n = summation of n+d, where d
so final answer = 3n*E(n)/2 + n*E(n)/2 = 2n*E(n)

Using here, n=1125, so you can easily find out the answer now
@naveenkrs said:
your answer will never match if n is odd.try this question - Sum of all positive integers less than 2250 and relatively prime to 1125.
you are not getting my point i said prime factors of 150 = prime factors of 300 and are same i.e. 2 , 3 , 5

but here prime factors of 2250 = x , 5 and 2
and prime factors of 1125 = a , b , 5 (2 is not here) so , that won't work here
@jain4444 said:
you are not getting my point i said prime factors of 150 = prime factors of 300 and are same i.e. 2 , 3 , 5 but here prime factors of 2250 = x , 5 and 2 and prime factors of 1125 = a , b , 5 (2 is not here) so , that won't work here
so why are you advocating a method that won't work in half of the cases?
@naveenkrs said:
so why are you advocating a method that won't work in half of the cases?
yaar i was just talking about that particular question i am not advocating anything...chill man this not a court house

Find the sum of all two digit positive integers which excede the product of their digits by 12.. share the approach

28 and 39 are the only 2 numbers i think..sum = 67

Did it by trial and error