Correct option is A
Let Total Work = 20 unit
Work done by A in 10 days
Efficiency of A =
Work done by A in 4 days = 2× 4 = 8 unit
Reamining work = 20-8 = 12 unit
It has to be completed by B in 9 days
Efficiency of B =
Total time taken by both of them together to complete the whole work =