Two workers A and B working together completed a job in 5 days. If A had worked twice as efficiently as he actually did, the work would have been completed in 3 days. To complete the job alone, A would require?

Answer: Option C

L.C.M. of Total Work = 15
One day work of A + B = $$\frac{15}{5}$$ = 3 unit/day
One day work of (2A + B) = $$\frac{15}{3}$$ = 5 unit/day
Now,
Assume A's efficiency is 2 units, B's is 1 unit.
So, it satisfies the equation of both cases
So, actual efficiency of A is 2 units/day
A alone can complete the work in
$$\begin{array}{rl}& =\frac{\text{Total work}}{\text{Efficiency}}\\ & =\frac{15}{2}\\ & =7\frac{1}{2}\text{days}\end{array}$$