Acomputer store bought a program at a cost of 20$ and sold it at a selling price of 24$ find the percent markup

The markup was 20%.

Markup percentage = price difference/original price

First, subtract the new price from the original. 
$24 - $20 = $4 

Second, divide the price difference by the original price. 
$4/$20 = .20 

Turn decimal into percentage by moving it to the right twice or multiplying by 100.

.20*100 = 20%

You can check it by multiplying the original cost by 20% then adding it to the original cost.

20*.20 = $4  ,   20+4 = $24 (current price)