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Order does not matter , so we ll choose the factors i.e. use combinatorics ans = 5C2 / 7C2. D

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Shamitjain
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If two distinct positive divisors of 64 are randomly selected, what is the probability that their sum will be less than 32?

A. 1/3
B. 1/2
C. 5/7
D. 10/21
E. 15/28

This is good question.

I did this in a slightly different way:

Total Factors of 64 are 64 , 1 , 32, 2 , 4 , 16 , 18

Sum of factors less than 32 - 1 , 2 , 4 , 16 , 18

therefore total no. of ways you can pick the first factor = 5/7 (as the same or either two numbers will be < 32)
Second number = 4/6 (since one number is already selected in the first fraction.

5/7 * 4/6 = 20/42 = 10/21 And since in selection AB = BA , we do not need to multiply by 2

Shamitjain I am no expert but you've asked down the thread so responding here, yes the approach is correct. 18 is not a factor of 64 though (I am 99% sure thats a typing error so you're sorted!)
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goodyear2013
If two distinct positive divisors of 64 are randomly selected, what is the probability that their sum will be less than 32?

A. 1/3
B. 1/2
C. 5/7
D. 10/21
E. 15/28

This is good question.

We need to find the number of factors of 64. We can do that by doing prime factorization, adding 1 to the exponent on prime factors and multiply, (if we have more than one prime factor base). 64=2^6, so there are 7 factors of 64. Doing a quick factor tree we get 2*32--->2*2*16....2*2*2*8. We can see that 16 is too far away to get us 32 so only 32 and 64 will get us there. So, the number of ways we can pick two factors >=32 are by picking 32 and any of the other 5 factors besides 64 and 32, by picking 64 and any of the other factors, or by picking 32 and 64. that gives us 5+5*1 = 11 factors that get us >=32. Now, the number of ways to pick 2 factors is 7C2=21.

So the probability two factors WILL BE >= 32 = 11/21, so the probability two factor will be <32 = 1-11/21 = 10/21. OE IS D
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Quote:
Figure out a similar pattern for powers of 3.
Sum of all factors of 3, less than \(3^n\) = \(\frac{3^n - 1}{2}\)
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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
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