Math chuckles

All credit here goes to a student of mine who brought this in...

The question: A mother is twenty-one years older than her son. In six years, the mother will be five times as old as her son. Where is the father?

The proof: - highlight below to read

Let Mom's age = X
Let Son's age = Y

X = Y+21 (first statement)
X+6 = 5(Y+6)

Via substitution...
Y+21+6 = 5(Y+6)
Y+27 = 5Y+30
-3 = 4Y
-3/4 = Y

So, the son's age is -3/4 years...or -9 months...