A can do a piece of work in 8 hours. B can do same piece of work in 12 hours. A starts working at 9AM and they worked on alternative hours. At which time work will be finished?

Sure, let's solve this step by step:

1. First, we need to find out how much work A and B can do in an hour. A can finish the work in 8 hours, so A can do 1/8 of the work in an hour. Similarly, B can do 1/12 of the work in an hour.

2. Now, let's see how much work they can do together in two hours (since they are working in alternate hours). In the first hour, A does 1/8 of the work, and in the second hour, B does 1/12 of the work. So, in two hours, they can do 1/8 + 1/12 = 5/24 of the work.

3. Now, we need to find out how many two-hour blocks they need to finish the work. Since they can do 5/24 of the work in two hours, they will need 24/5 two-hour blocks to finish the work. 24/5 = 4.8, so they will need 5 two-hour blocks to finish the work, because they can't work in fractions of an hour.

4. So, they will finish the work in 5 * 2 = 10 hours.

5. A starts working at 9AM, and they work for 10 hours, so the work will be finished at 7PM.