Quant Boosters - Anubhav Sehgal, NMIMS Mumbai - Set 1
Q26) In how many ways can 7^17 be written as a product of 3 natural numbers?
abc = 7^17
a = 7^x , b = 7^y , c = 7^z
abc = 7^(x + y + z) = 7^17
x + y + z = 17 => C(17 + 3 - 1,3 - 1) = C(19,2) non negative integral solutions
But this includes solutions where any two of a,b,c are equal and permutated in 3!/2! = 3 ways while
we need ways to write it as product of 3 natural numbers hence should be counting them only once.
2a + b = 17 => 9 solutions
Unordering the solutions : (C(19,2) - 3*9)/3! + 9 = 33 ways ; +9 to include them once.
Q27) Sum of 20 distinct numbers is 801.What is their minimum possible LCM ? (source TG)
Firstly it is a bit of hit and trial question..
LCM will be minimum when either maximum possible numbers are equal or when they are all factors of the same number as we need LCM,the Lowest common multiple to be minimum.
In any other case there will be extra power of primes or new primes introduced increasing the LCM.
Part I :
If the numbers may or may not be distinct
We try to bring them as close to each other(or equal as possible)
801 = 19 * 42 + 3
So we take 19 numbers as 42 and 20th number as its factor so that LCM stays minimum.
Part II :
When we need all numbers to be distinct
Now that we can't have any numbers equal we try to find a number which has at least 20 factors whose sum is 801.
Here there is a bit of hit and trial looking for numbers with 21,22,23 and eventually 24 factors to reach 360.
And then eliminating some of its factors to get our required sum of 20 numbers(out of those 24 factors) to be 801.
And their LCM will be 360 as they all were its factors and had no other lower common multiple even after removal of the said four extra factors.
LOTS TO READ and UNDERSTAND but there is no alternate way for this. It is just for conceptual clarity and practice.
Q28) Let M and N be single digit numbers. If product 2M5 * 13N is divisible by 36.How many ordered pairs of (M,N) are possible?
2M5 is odd
=> 13N is divisible by 4
=> N = 2 or 6
i) 2M5 * 132
=> 2M5 is divisible by 3
=> M = 2, 5, 8
ii) 2M5 * 136
=> 2M5 is divisible by 9
=> M = 2
Q29) The number of integer solutions of the equation x^2 + 12 = y^4.
x^2 + 12 = y^4
y^4 - x^2 = 12
(y^2 + x)(y^2 - x) = 12 = 1 * 12 = 2 * 6 = 3 * 4
No solutions for 1 * 12 and 3 * 4 factor pairs as RHS(=12) is even hence we need just even * even factor pairs.
(y^2 + x)(y^2 - x) = 2 * 6
y=+/- 2 and x=+/-2
Q30) The number 2006! is written in base 42. How many zeroes are there at the end?
42 = 2 * 3 * 7
No of zeroes in 2006! will be the highest power of 7 in 2006!
[2006/7] + [2006/49] + [2006/343] = 331
as 42 in base 42 will be 10 and will be the provider of trailing zeroes