To raise money for new uniforms, the band boosters sell t-shirts and hats. the cost and sale price of each item is shown. the boosters spend a total of $2,000 on t-shirts and hats. they sell all of the merchandise, and make $3375. how many t-shirts did they sell?

Found the problem:
Item     Cost       Sales Price
T-shirt    6                10
hats       4                  7

Let x = t-shirts, y = hats

6x + 4y = 2,000
10x + 7y = 3,375

4y = 2000 - 6x
y = 2000/4 - 6x/4
y = 500 - 3x/2

10x + 7y = 3,375
10x + 7(500 - 3x/2) = 3,375
10x + 3500 - 21x/2 = 3,375
10x - 10.5x = 3,375 - 3500
-0.5x =  -125
-0.5x / -0.5 = -125/-0.5
x = 250 shirts

They will need 250 shirts.