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?
Give me a detailed solution. 
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