[Question]: A vampire drains three people a week. His best friend is a werewolf who eats five people a week, but only fresh ones, never drained ones. How many weeks will a village of 72 people last them both?
[Answer]: The vampire and werewolf need 3 + 5 = 8 people a week. A village of 72 people will last them both 72 / 8 = 9 weeks.
So the answer is 9.

[Question]: Alice's white water rafting class is composed of 40 students and 10 instructors, including herself. She has 20 life vests on hand. 20% of her students are bringing life vests. How many more life vests will Alice need to get so that entire class has one?
[Answer]: 20% of her 40 students will bring their own life vest so .20 * 40 = 8 students will bring their life vests. Alice has 20 life vests and 8 students will bring there's so that's 20 + 8 = 28 life vests. There are 40 students and 10 instructors total so that's 40 + 10 = 50 people in the class. If there are 28 life vests and she has 50 people then she needs to find 50 - 28 = 22 more life vests.
So the answer is 22.

[Question]: Ralph is going to practice playing tennis with a tennis ball machine that shoots out tennis balls for Ralph to hit. He loads up the machine with 175 tennis balls to start with. Out of the first 100 balls, he manages to hit 2/5 of them. Of the next 75 tennis balls, he manages to hit 1/3 of them. Out of all the tennis balls, how many did Ralph not hit?
[Answer]:
Out of the first 100 balls, Ralph was able to hit 2 / 5 of them and not able to hit 3 / 5 of them, 3 / 5 x 100 = 60 tennis balls Ralph didn't hit. Out of the next 75 balls, Ralph was able to hit 1 / 3 of them and not able to hit 2 / 3 of them, 2 / 3 x 75 = 50 tennis balls that Ralph didn't hit. Combined, Ralph was not able to hit 60 + 50 = 110 tennis balls Ralph didn't hit.
So the answer is 110.