a. Let's start with "the sum of one-half and one-third." Sum tells us to add. This would translate to 1/2 + 1/3. Now, it's telling you to find 1/5 of that. So you multiply this by 1/5.
Your final expression is (1/5)(1/2 + 1/3)
b. "the sum of nine and twelve" is more obvious; 9 + 12. Now, multiply it by "two and one-half"
Your final expression is (2 1/2)(9 + 12)
c. "difference between 1 1/2 and 3/4." Difference tells us to use subtraction. 1 1/2 - 3/4. "twenty-four divided by" tells you that that part of the equation belongs in the denominator.
Your final expression is 24 / (1 1/2 - 3/4)