Explanation:

- A and B together do work in 30 days. So, they complete 1/30 of the work in a day.

- Together, they work 6 days. So, they finish 6/30 or 1/5 of the work in 6 days.

- B completes the remaining work alone in 36 days, which is 4/5 of the work.

- If B completes 4/5 of the work in 36 days, he completes 1/5 of the work in 9 days.

- So, B alone completes the entire work in 45 days (9 x 5 = 45).

- The work done by B in a day is 1/45.

- The combined work per day is 1/30. Therefore, A's work per day is 1/30 - 1/45 = 1/90.

- A alone can complete the entire work in 90 days.