###Human: 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?, ### Assistant: He contacted 1-.4=.6 of his friends
So he contacted 100*.6=60 people
So he removes 60/2=30 people
So he has 100-30=70 friends left 
 So the answer is :  70