Riya alone can complete a piece of work in 6 days while Siya alone can do the same work in 9 days. If they both work together, then how many days will be required to finish the work?

18/5 days

                    Riya  :  Siya 

Time taken      6    :    9

Efficiency         3    :   2

Total work = LCM of 6, 9 = 18

 Total efficiency = 3 + 2 = 5

Time taken to complete the work together = $\frac{total\; work}{efficiency}$

                                    = $\frac{18}{5}$ days