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=
<gadget id="calculator">1-.4</gadget>
<output>0.6</output>
.6 of his friends.
So he contacted 100*.6=
<gadget id="calculator">100*.6</gadget>
<output>60</output>
60 people.
So he removes 60/2=
<gadget id="calculator">60/2</gadget>
<output>30</output>
30 people.
So he has 100-30=
<gadget id="calculator">100-30</gadget>
<output>70</output>
70 friends left.

<result>70</result>