If A does 1/n work in one day: A finish the work in how many days

A can finish a work in 12 days and B can do it in 15 days. ’After A had worked for 3 days, B also joined A to finish the remaining work. In how many days, the remaining work will be finished?

1. A and B together can finish a work in 15 days. If If A alone can finish this work in 24 days, B alone can do this work In how many days?