Mahesh, Suresh and Raju together can complete a work in 18 days. Mahesh and Suresh together can complete the same work in 72 days. Raju alone can finish the same work in how many days?

24 days

Total Work = LCM of 18, 72 = 72

'

Efficiency of Raju = eff(Mahesh+Suresh+Raju) - eff(Mahesh+Suresh)

                          = 4 - 1 = 3

Time taken by Raju alone to complete the work = $\frac{total\;work}{efficiency\;of\;Raju}$

                         = $\frac{72}{3}$ = 24 days