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