Percentages

1) Never Add or Subtract Percentage Changes

When a quantity is increased or decreased multiple times, you can't add the percentages together. If a stock went up 10% on Monday and then went down 10% on Tuesday, its final value is not equal to 100% of its starting value.

2) Start with 100 and work through the percent changes

In order to properly determine the final percentage value of a stock that went up 10% on Monday and went down 10% on Tuesday, assume a $100 starting value.

1. Apply a 10% increase on Monday by multiplying 100 by 1.1:
   100 * 1.1 = 110
2. Apply a 10% decrease on Tuesday by multiplying 110 by 0.9:
   110 * 0.9 = 99
3. Therefore, after an identical percentage increase and decrease you only have $99.

3) Practice:

Sally bought a bike at a garage sale for 50% less than it originally sold for in the store. After fixing the bike up, Sally sold the bike to her friend Tom for 30% more than she had paid. The price Tom paid was what percent less than the bike's original store price?

1. 20%
2. 25%
3. 30%
4. 35%
5. 40%