Tom plants a tree that is 1 year old and 5 feet tall.  It gains 3 feet per year.  How old is it when it is 23 feet tall?
It has grown 23-5=<T>subtract(23, 5)=18 feet
So it is 18/3=<T>divide(18, 3)=6.0 years older
That means it is 6+1=<T>add(6, 1)=7 years old
