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=<<4*4=16>>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= <<16-4=12>>12 years old
#### 12