7.RP.A.3
Use proportional relationships to solve multistep ratio and percent problems. Examples: simple interest, tax, markups and markdowns, gratuities and commissions, fees, percent increase and decrease, percent error.
Solve percent problems, where the task is to determine the whole, the part, or the percent. Here's an example of finding the whole:
40% of what number is 80?
Here's an example of finding the part:
What is 4% of 110?
Some finding the part problems can be done mentally. For example:
What is 15% of 300?
It's easy to find 10% of 300. That's just 30. So 5% must be half that, 15. Therefore, 15% of 30 must be 30 + 15 = 45. Finally, here's an example of finding the percent:
What percent of 315 is 126?