A and B completed a work together in 5 days. had A worked at twice the speed and B at half the speed, it would have taken them four days to complete the job. How much time would it take for A alone to do the work?

Answer: Option A

Assume work to be done 100%.
First case,
A + B = $$\frac{100}{5}$$ = 20% work done per day -------- (1)
Second case,
2A + $$\frac{\text{B}}{2}$$ = $$\frac{100}{4}$$ = 25% work done per day ------ (2)
On solving equation (1) and (2), we get
A = 10 days