Question

Fred buys a video game disk for 4$. there was a discount of 20%. what is the sales price?

Answer 1

The answer is this: $3.60

4 x 10 = 40 - 0.4 = 3.6 

Answer 2

According to the conditions, you can get the formula as:
4*(100%-20%)
=4*80%
=4*0.8
=$3.6