Given : Raju can finish a piece of work in 20 days Raju worked for 5 days Jakob finished the remaining work in 15 days Concept used : The efficiency of work = 1/(number of days to finish work) Solution : Raju worked for 5 days So, the remaining work will be finished in 15 days by Raju Jakob can finish the same piece of work in 20 days The efficiency of Raju = The efficiency of Jakob One day work of Raju =1/20 One day work of Jakob =1/20 One day work working together =(1/20)+(1/20) ⇒ One day work working together =(1+1)/20 ⇒ One day work working together =2/20 ⇒ One day work working together =1/10 ∴ The number of days they both can finish work together is 10 days.