First, find the cost of the shirts by dividing the total cost by the total amount of shirts:
[tex]\frac{3600}{200}=18[/tex]Since each shirt costs $18 and the selling price is $20, divide 20 by 18 and multiply by 100 to find the percent markup:
[tex]\frac{20}{18}\cdot100=111.11[/tex]Since the store sells the shirts with an increase of 11.11%, then the percent markup is 11.11%.