Explanation:

- Mohan's work rate is 1/10 of the work per day.

- Sohan's work rate is 1/15 of the work per day.

- When working together, they complete (1/10 + 1/15) = 1/6 of the work per day.

- After 3 days of working together, they've completed 3/6 = 1/2 of the work.

- Half of the work is left after Mohan leaves.

- Sohan now does the remaining work alone at 1/15 per day.

- Sohan will take (1/2) / (1/15) = 15/2 days to finish.

Option 1: 15/2 days