A is able to do a piece of work in 10 days and B can do the same work in 15 days. If they can work together for four days, what is the fraction of work left?

It is given that A can do a piece of work in 10 days and B can do the same work in 15 days.

Let the total work be LCM of (10, 15) = 30 units

Amount of work done by A in one day = $$\ \frac{\ 30}{10}$$ = 3 units

Amount of work done by B in one day = $$\ \frac{\ 30}{15}$$ = 2 units

Amount of work done by A and B together in one day = 3+2=5 units

So, together will complete 5*4 units i.e 20 of work in four days.

The amount of work left out = 30-20=10 units

The fraction of work left out = $$\ \frac{\ 10}{30}$$ = $$\ \frac{\ 1}{3}$$

D is the correct answer.