P, Q and R can complete a work in 24, 6 and 12 days respectively. The work will be completed in ______ days if all of them are working together ?

Work done by P in 1 day = 1/24
Work done by Q in 1 day = 1/6
Work done by R in 1 day = 1/12
Work done by P,Q and R in 1 day = 1/24 + 1/6 + 1/12 = 7/24
=> Working together, they will complete the work in 24/7 days = 3 3⁄7 days