The identity 3*5=15 quickly leads to more mathematics:
various algebraic identities, an infinite product,
Fibonacci numbers, the Golden Ratio, Mersenne primes,
sums of squares, complex numbers, and quaternions.
I've posted an essay on this at
<http://www.gbbservices.com/math/threefive.html>.
Hope you enjoy it,
Walter
P.S. The programmers on the list might also be interested
in "Personal Bug Tracking with Mantis on Windows" at
<http://www.gbbservices.com/prog/mantis.html>