A is twice as good a workman as B and together they finish a price of work in 14 days

Explanation
Let us assume that A takes x days to complete the work so by the given condition B will take 2x days to complete the work
Now the time taken by A to complete the work will be and similarly the time taken by B will be
Total time taken is 14 days.
So the equation is formed
1/x+1/2x=1/14
(2+1)/2x=1/14
3/x=1/7
x=21
Final Answer
The correct option is option D i.e. x=21