If I'm 4 times older than Billy currently, and Billy is 4 years old, how old was I when Billy was born?
I would currently be 4*4=<T>multiply(4, 4)=16 years old, since I'm 4 times older than Billy's age of 4.
Therefore, in order to find my age at Billy's birth we'd simply subtract Billy's age of 4 from my age, meaning I was 16-4= <T>subtract(16, 4)=12 years old
