Madhya Pradesh Police Exam Model Paper 6

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 $=\left(1/20\right)+\left(1/20\right)$
⇒ One day work working together $=\left(1+1\right)/20$
⇒ 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.