P, Q and R can finish a work in 5 days, 10 days, and 15 days, respectively, working alone. P and Q works on first day, P and R works on second day and P and Q works on third day and son on till the work is completed. In how many days the work will be completed?

A

$$\frac{13}{2}$$

B

$$\frac{9}{2}$$

C

$$\frac{7}{2}$$

D

$$\frac{5}{2}$$