Raju can finish a piece of work in 20 days. He worked at it for 5 days and then Jakob alone finished the remaining work in 15 days. In how many days can both finish it together?

Let the total work = W

Number of days required for Raju to complete the work = 20 days

$$=$$>  Work done by Raju in 1 day = $$\frac{W}{20}$$

$$=$$>  Work done by Raju in 5 days = $$\frac{5W}{20}=\frac{W}{4}$$

Remaining work = $$W-\frac{W}{4}=\frac{3W}{4}$$

$$\therefore\ $$Work done by Jakob in 15 days = $$\frac{3W}{4}$$

$$=$$>  Work done by Jakob in 1 day = $$\frac{3W}{60}=\frac{W}{20}$$

$$=$$>  Work done by Raju and Jakob in 1 day = $$\frac{W}{20}+\frac{W}{20}=\frac{2W}{20}=\frac{W}{10}$$

$$=$$>  Number of days required for both Raju and Jakob to complete the work = $$\frac{W}{\frac{W}{10}}=10$$ days

Hence, the correct answer is Option C