Quant Boosters  Anubhav Sehgal, NMIMS Mumbai  Set 2

Solve and Share your method for this one

Q4) X is a set of all natural numbers with 4 factors such that sum of all its factors excluding number itself is 31. Find the sum of all such numbers

Case 1 : N = a * b
Factors will be 1,a,b,ab
1 + a + b = 31
a + b = 30
a,b = 7,23 ; 11,19 ; 13,17 => N = 161,209,221Case 2 : N = a^3
Factors : 1,a,a^2,a^3
1 + a + a^2 = 31
a(a + 1) = 30
a = 5 is the only case
N = 125Sum = 716

Q5) Find the remainder when C(58,29) is divided by 29.

C(2n,n) = C(n,0)^2 + C(n,1)^2 + .. + C(n,n)^2
For our question,
C(58,29) mod 29
= C(29,0)^2 + C(29,1)^2 + .. + C(29,29) mod 29
All terms except first and last are divisible by 29.
Remainder = C(29,0)^2 + C(29,29)^2
= 1 + 1

Q6) How many numbers from 1 to 100 are NOT divisible by 2 or 3 or 5 or 7 ?

105 * 1/2 * 2/3 * 4/5 * 6/7  {101,103} = 22
Why 105 ?
Step 1 : Find numbers not divisible by 2,3,5 or 7 by simple Euler number's application.
Step 2 : Remove numbers like 103 which being not divisible by 2,3,5 or 7 were removed in euler but do not belong to our range of 1100 for which answer is to be calculated.
105 Chosen as it is the closest(to100) multiple of LCM of uncancelled denominators 5,7

Q7) HCF of 2 numbers is 12 and their sum if 144. Find the maximum value of their product

12(a + b) = 144
a + b = 12
As near to each other as possible with they being coprime to each other.
5, 7
Max product = (12 * 5) * (12 * 7) = 60 * 84 = 5040.

Q8) Highest power of 8 in 17! + 18! + 19! + . . . + 100!?

17!(1 + 18 + 18 * 19 + ... 18 * 19 * 20 * ..100)
17!(1 + even)
17! * odd
Find power of 2 and subsequently 8 in 17!
Power of 2 : 8 + 4 + 2 + 1 = 15
Power of 8 : [15/3] = 5

Q9) What is the highest power of 3 available in the expression 58!  38!

38!(58 * 57 * .. * 39  1)
Power of 3 in 38! = 12 + 4 + 1 = 17
Power of 3 inside bracket = 0
Highest power to divide 58!  38! = 17

Q10) Remainder when 26^57 is divided by 29?

E(29) = 28
26^57 mod 29
26^56 * 26 mod 29
26

Q11) How many scalene triangles are there for which the lengths of all sides are integers & the perimeter is 24 cm?

For this particular case, [(n  6)^2/48] where [.] nearest integer function.
[(24  6)^2 / 48 ] = 7

Q12) The number of 4letter words that can be formed out of the letters of the word FIITJEE is

F,I2,T,J,E2
All different : 5c4 * 4! = 120
2 same 2 different : 2c1 * 4c2 * 4!/2! = 144
2 same 2 same : 2c2 * 4!/2!2! = 6
Total : 270

Q13) Find the sum of the first 125 terms of sequence 1, 2, 1, 3, 2, 1, 4, 3, 2, 1, 5, 4, 3, 2 ...