I know this is really simple, but I am drawing a complete blank. Thanks!

**Q**: You have a coupon for 10% off the price of a CD. You pay $18 for a CD. How much would you pay if you did not have a coupon?

I know the answer is $20, but I cannot figure out the equation. Thank you!

Finally figured it out.

90/100=18/x

cross multiply

90x=1800

x=20