Mark realizes that a lot of the friends on his friends list are people he doesn't talk to anymore.  He keeps 40% of his friends list and then contacts the rest.  Of those only 50% respond.  He removes everyone who did not respond.  If he had 100 friends how many does he have left after the removal?
He contacted 1-.4=<T>subtract(1, 0.4)=0.6 of his friends
So he contacted 100*.6=<T>multiply(100, 0.6)=60.0 people
So he removes 60/2=<T>divide(60, 2)=30.0 people
So he has 100-30=<T>subtract(100, 30)=70 friends left
