If we check the possible outcome it is actually quite simple.
(4, -5)
We buy the required 3 pairs of jeans but you cannot buy negative amount of something so that is wrong
(4, 5)
We buy the required 3 pairs of jeans but the total price is 4*$15 = $60 + 5*$9 = $45 = $105 which is $15 too much
(2, 4)
We do not buy the required pairs of jeans, therefore this answer is incorrect as well
Finally we are left with (4, 2)
- We buy the required 3 pairs of jeans
- And the total price is $78, $12 less than the maximum amount of money we can spend
Bonus: If we wanted, we could buy another shirt and still be left with $3
