A and B together can do a work in 6 days. They worked for 3 days and then C joined them. As a result the work could be done in 5 days. If A, B and C work together from the beginning, then in how many days will the work be done?

4

A and B together can do a work in = 6 days

One day's work of A and B is = 1/6

Three day's work = 1/2

so, remaining 1 - 1/2 = 1/2 work is done by A + B + C in = 2 days

Then, whole work done by A + B + C in = 4 days