Percentage decrease

Percentage decrease tells us how much a number dropped. Let's learn it on an example: 12 kids in our class did not know how to calculate percentages. Now that we have practiced it a bit, only 9 kids cannot do it. What's the percentage decrease? First, tell me what's the decrease. 12 - 9 is 3. Great. So what's the percentage decrease? Some of us already know the formula for percentages:
Percentage = 100 * numerator / denominator, where numerator is our decrease (3) and denominator is our original value (12). So the formula for percentage decrease is:
Percentage decrease = 100 * decrease / original value
So in our example percentage decrease = 100 * 3 / 12 = 300 / 12 = let me get my calculator... 25! The number of kids that don't know how to calculate percentages dropped by 25 percent.

One more example. People say arms is a good market to invest in. Some people are just that: weird, but let's roll with it. Spud guns were 130 dollars last week. This week there's a huge sale and they're just 104. The blurry sign says XX% off... we just can't see how great the sale is. Let's employ our newly acquired skills to calculate it by ourselves! We've already established the formula:
Percentage decrease = 100 * decrease / original value. The decrease is 130-104 = 26, so:
Percentage decrease = 100 * 26 / 130 = 2600 / 130 = 260 / 13 = 20. The sign says "20% off!".

Now come up with some problems and use this tool to solve them:

Calculating percentage increase is very similar. In fact, they're both just a specific form of percentage change. You know how to calculate percentage already, do you?