Butt Koko, this is the solution:
Original price = 19.99
Sale price = 15.99
Discount = 19.99 - 15.99 = 4.00
We can use Direct Rule of Three to calculate the percent of discount, this way:
Price Percent
19.99 100
4.00 x
___________________
4 * 100 = 19.99 * x
400 = 19.99x
x = 400/19.99
x = 20.01
The approximate percent discount on the frisbee is 20%