Pipe A fills in 6 hours. Pipe B in 12 hours. Combined time = ?

Explanation

The correct answer is B: 4 hours. To find the combined time for Pipe A and Pipe B to fill the tank together, calculate their rates of work: Pipe A's rate is 1/6 (since it fills the tank in 6 hours) and Pipe B's rate is 1/12. Add these rates together to get their combined rate: 1/6 + 1/12 = 2/12 + 1/12 = 3/12 = 1/4. The combined rate of 1/4 means the pipes together fill 1/4 of the tank in 1 hour, so they will fill the whole tank in 4 hours. Option A is incorrect because it assumes a faster filling rate. Option C underestimates the combined efficiency, and Option D is the time for Pipe A alone, not the combined time.