A and B can do a piece of work in eight days and B alone can do it in 16 days. In how many days, can A do it alone?

Correct option is D

Given:
A and B can do a piece of work = 8 days
B alone can do it = 16 days.
Solution:
A and B 1 day work =  $$\frac{1}{8 }$$​unit
B’s 1 day work = $$\frac{1}{16}$$​ unit
Then, for A’s one day work
A‘s one day work =  $$\frac{1}{8}-\frac{1}{16}=\frac{ (2-1)}{16}=\frac{1}{16}$$ 

A’s one day work =  $$\frac{1}{16}$$ unit ​
Therefore, A alone do the total work in 16 days.
Alternate Method:

A+B’s efficiency = 2 unit
B’s efficiency = 1 unit
Now, for A’s efficiency = A+B - B = 2 – 1 = 1
A’s efficiency is 1 unit.
So, A can do 16 unit work in = $$\frac{16}{1}$$​ = 16 days