Answer
$78\ hours$
Work Step by Step
Step 1. Use the table given in the exercise, the rate from B would be $\frac{1}{2x}$ and the part of job completed from B would be $26(\frac{1}{2x})$.
Step 2. Together, the total parts should be 1, we have $26(\frac{1}{x})+26(\frac{1}{2x})=1$ or $\frac{1}{x}+\frac{1}{2x}=\frac{1}{26}$
Step 3. Solve the above equation: $\frac{3}{2x}=\frac{1}{26}$, thus $x=39\ hrs$
Step 4. It takes B $2\times39=78\ hours$ alone.