Asked: If two distinct positive divisors of 64 are randomly selected, what is the probability that their sum will be less than 32?
64 = 2^6
Divisors is of the form = 2^n : where 0<=n<=6
Divisors of 64 = {1,2,4,8,16,32,64}
Sum of 2 distinct divisors = 2^n + 2^m : where n & m are positive integers in the range [0,6] and are distinct; Say n<m
If n=1
If m = {32,64}
Sum of divisors > 32
Total cases = 6
Favourable cases = 6-2 = 4
If n=2
If m = {32,64}
Sum of divisors > 32
Total cases = 5
Favourable cases = 5-2 = 3
If n=4
If m = {32,64}
Sum of divisors > 32
Total cases = 4
Favourable cases = 4-2 = 2
If n=8
If m = {32,64}
Sum of divisors > 32
Total cases = 3
Favourable cases = 3-2 = 1
If n=16
If m = {32,64}
Sum of divisors >= 32
Total cases = 2
Favourable cases = 2-2 = 0
If n=32
If m = {64}
Sum of divisors >= 32
Total cases = 1
Favourable cases = 1-1 = 0
Total cases = 6+5+4+3+2+1 = 21
Favourable cases = 4+3+2+1 = 10
Probability that their sum will be less than 32 = 10/21
IMO D