You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

[EX Q]: Question: One water pump can fill half of a certain empty tank in 7 hours. Another pump can fill half of the same tank in 7.5 hours. Working together, how long will it take these two pumps to fill the entire tank?

 Option A: 7 7/13

 Option B: 1 5/8

 Option C: 3 1/4

 Option D: 7 7/29

 Option E: 3 1/2
[EX A]: Option D

[EX Q]: Question: Find the value of y from (12)^3 x 6^4 ÷ 432 = y?

 Option A: 3451

 Option B: 3467

 Option C: 3891

 Option D: 4601

 Option E: 5184
[EX A]: Option E

[EX Q]: Question: If a and b are prime numbers, which of the following CANNOT be the sum of a and b?

 Option A: 5

 Option B: 9

 Option C: 23

 Option D: 16

 Option E: 13
[EX A]:
Option C