3. how could you check the answer to 27 – 15 = 12 using addition?
a. 15 – 12 = 27
b. 12 + 15 = 27
c. 27 + 12 = 15
d. 27 + 15 = 12

The answer would be the second choice, B. 12+15=27.

The answer could not be the first choice, as the difference between 15 and 12 cannot be greater than either of the numbers.
The answer could not be the third choice, as the sum of 27 and 12 should not be less then 27.
The answer could not be the fourth choice, for the same reasoning that I used for the third choice being wrong. The sum of 27 and 15 cannot be smaller than either number.