Question: 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? Think carefully first, then make a decision:
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.