The second man could not answer after knowing the Sum total of 3 numbers and also their Product (his age).

This means that there exists two sets of numbers having two common = meaning 1 set of twins.

all possible combinations are;

1,1,11 = 11

2,2.9 = 36

3,3,7 = 42

4,4,5 = 40

5,5,3 =75

6,6,1=36

Even with this knowledge he could not answer because possible combination can be either

1) 2,2,9 = 36

OR

2) 6,6,1 = 36

Once the first man said "my oldest son weighs 91 pounds"

the second man could conclude that the correct combination is 2,2,9

OR say **oldest Son of age 9 and then a set of twin of 2 years of age.**