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?
Let's think step by step below.
70
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.

.

The answer: 70