X and Y can do a piece of work in 20 days and 12 days respectively. X started the work alone and then after 4 days Y joined him till the completion of the work. How long did the work last?

- 6 days
- 10 days
- 20 days
- 15 days

**Solution**

Let total work that has to be done is 120 units (common multiple of 20 and 12).

In one day, X can do 6 units and Y can do 10 units of work. Thus, they both can do 16 units of work in a day.

In 4 days, X did (6 * 4) or 24 units of work.

After 4 days, work left to be done = 120 - 24 = 96.

Now, both X and Y work till the completion of work. They can do 16 units of work in one day. To complete at 16 units per day, 6 more days will be more required.

Total days = 4 + 6 = 10

**The correct option is B.**