The student council sold 661 T-shirts, some at $10 and some at $12. When recording the number of T-shirts they had sold at each of the two prices, they reversed the amounts. They thought they made $378 more than they really did. How many T-shirts actually were sold at $10 per shirt?
+36916 x = 10 dollar shirts
661 - x = 12 dollar shirts
10x + 12 (661-x) + 378 = 12x + 10(661-x)
10x +7932 - 12x + 378 = 12x +6610 -10x
x = 425 shirts @ $10
