Susan's math teacher assigned x homework problems on Monday. On Tuesday, she assigned 12 more problems. Over the two days, a total of 20 homework problems were assigned. Which equation could be used to find x, the number of problems assigned on Monday?
A.
x - 12 = 20
B.
20 + x = 12
C.
12x = 20
D.
x + 12 = 20