Solution
For this case we can do the following
We have 2 addition problems for every 3 subtraction problem
Then the total of combinations are:
B. 6 addition problems and 9 subtraction problems (since 9/6= 3/2)
C. 16 addition problems and 24 subtraction problems since (24/16 = 3/2)
D, 8 addition problems and 12 subtraction problems since (12/8 = 3/2)