A shirt costs 20$ if it was on sale for 75 percent off what is the discount

You could do x/20 =75/100 and then cross multiply
Then you would have 100x=75*20 and if you simplify all the way you would get x=15
So the discount would be $15

Hope this helps :)

If the shirt cost $20 before discount:
convert 75% to decimal- so divide by 100; you will get 0.75
then you multiply 20 and 0.75
20 x 0.75 = 15
the discount is $15
(however, remember that the new shirt price would be $5)

if the shirt cost $20 after discount:
we know that 100% is the whole, and 75% is the discount, so this means that the leftover 25% = $20
so using proportions,
25/100=20/x, cross multiply & simplify you would get x = 80
to find the discount, subtract the cost from the original price and get 60
the discount was $60
(however remember the original price of the shirt was $80)