I would use T-Shirts as sweatshirts make you no profit. 20 hours = 1200 / 10 (cuz it takes 10 min) = 120.
So you can make 120 shirts. 120 x4(for the cost of supplies) =$ 480.
you would make profit of 120 x 6 = 720.
720 - 480 = 240.
You would make $240 profit of making 120 shirts