So let's say x is the money they made at 10 dollars(a) and 12(b). The two variables are for the number sold.
They reversed it so that it was 12(a) and 10(b).
Thus
12a +10b = 10a + 12b + 378
Some rearrangement:
12a +10b - 10a -12 b = 378
2a - 2b = 378
a-b = 189
So we sold 661 tshirts.
661-189= 472,/2 = 236
So b = 236 and a = 236 + 189 = 425.
425*10 + 236*12 = 7082
Flip around, 425* 12 + 236 * 10 = 7460.
7460- 7082 = 278.
Thus 425 at 10, 236 at 12.
if you don't understand anything feel free to ask.
Great solution, whymeissmart hehehe