We have the following:
To find the percent markup, we must calculate the profits, knowing that there are 300 shirts and they sell them for a total of $ 20
[tex]300\cdot20=6000[/tex]Now we subtract from what was initially spent and then divide by that same amount and multiplying by 100 we will know the percent markup
[tex]\frac{6000-3600}{3600}\cdot100=66.67\cong67[/tex]Which means that the percent markup is 67%