John is 10 years old. His sister is twice his age. When he is 50 years old, how old will his sister be?
His sister is 10*2= <T>multiply(10, 2)=20 years old
Their age difference is 20-10= <T>subtract(20, 10)=10 years
Therefore when he is 50 she will be 50+10= <T>add(50, 10)=60 years old.
