Answer
$16\ hrs$
Work Step by Step
Step 1. Use the table given in the exercise, the part of job completed by the faster pipe would be $12(\frac{1}{x})$, while the part of job completed by the slower pipe would be $12(\frac{1}{3x})$
Step 2. Together, the total parts should be 1, we have $12(\frac{1}{x})+12(\frac{1}{3x})=1$ or $\frac{1}{x}+\frac{1}{3x}=\frac{1}{12}$
Step 3. Solve the above equation: $\frac{4}{3x}=\frac{1}{12}$, thus $x=16\ hrs$
