3s + 4j = 360
1s + 3j = 220
This one we can solve using substitution, in that
s + 3j = 220 implies s = 220 - 3j (subtract 3j from each side).
Now substitute into the first equation:
3(220 - 3j) + 4j = 360. Solve for jackets.
660 - 9j + 4j = 360 [Distributive property]
660 - 5j = 360 [Add -9j + 4j]
Subtract 360 from each side, and add 5j to each side.
660 - 360 - 5j + 5j = 360 - 360 + 5j
300 = 5j
Divide each side by 5.
$60 = price of a jacket.
Three of these plus a shirt runs $220.
3(60) = 180
180 + s = 220
180 - 180 + s = 220 - 180 [Subtract 180 from each side]
s = 40
Now we double check.
3(40) + 4 (60) = 120 + 240 = 360. This satisfies the first equation.
We could check the second one too, but I'm satisfied.