What is the total cost if the shirt cost $40 and 20% off

You'd simply have to find 20% of $40. To find the percent of a number, you would change the percent into an actual decimal/number, and then multiply them both. So the percent would become 0.20.

40 × 0.20 = 8

So now you would subtract 8 from the cost of 40.

 40 - 8 = 32.

The shirt costs $32.