In four years, Suzy will be twice Mary's age then. If Suzy is 20 now, how old is Mary?
In four years, Suzy will be 20+4=<T>add(20, 4)=24
Then Mary will be 24/2=<T>divide(24, 2)=12.0
Then Mary is now 12-4=<T>subtract(12, 4)=8
