18 was added on the right side instead of subtracting
1) Since the expression states that
Q +18 = 22
Q +18-18 = 22 +18
Q=40 Wrong
2) We can see the mistake when the 18 was added on the right side rather than subtract from 22. So the correct would be like this:
Q +18 = 22
Q +18-18 = 22 -18
Q = 4 True!
3) Hence, the answer is
18 was added on the right side, when it should subtract