One man can paint a house in 10 days and another man can do it in 15 days. If they work alone on alternate days, starting with the first man on day 1, they complete the job in X days. Find the value of X.

If P and Q together can complete a piece of work in 15 days and P alone can do it in 45 days, in how many days can Q alone complete that work?

A can lay a railway track between two stations in 16 days and B can do the same job in 12 days. With the help of C, they finished the job in 4 days. In how many days can C alone do the job?