Did you deduce the building number just because he "still didn't know" and 14 was the only double? Isn't it possible he didn't know because of other reasons, rather than the available options. Maybe he just couldn't figure out all the options of factors of 72, maybe he couldn't literally see the building number :)

Another thing, because the problem gave over 20 years, we can reason that it hasn't been over 30 years. If it was a trick question we couldn't propose that, but the significance of choosing 20 as opposed to 10 or 30 is a significant statement given the nature of the problem. Thus, you can rule out oldest daughters greater than 21.

If it had been 20 years since he last saw his friend, he would have known about the eldest daughter. In the options given, the 36 y/o would have been 16 when they last saw each other. I chose 21 because if they were truely friends the one would have known the other's wife was pregnant since he last saw him, thus giving that extra year to know about it, without seeing his friend.

Because the problem says "over" 20, you might want to change that 21 i chose above to something like 23 (reason to follow). The next significant number the problem would have said is "over 25." So the actual time should be between 20 and 25. Due to human psychology you can rule out 24 because a person would have said "almost 25" rather than "over 20" - you might argue the same for 23. So, given that assumption, you could reason the actual years since the two have last seen one another is between 21 and 23 :) Given this information, you can modify your formula to rule out any girls that are older than that.

Thinking of these other factors is important in math modeling. What if the "oldest playing piano" was "the oldest can now vote?" That 24 and 36 become more significant in eligibility, which we have not selected by reason albeit conjectural and not empirical.

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