Ravi and Mohan together can complete a task in 3 days. Ravi alone can complete can complete the same task in 7 days. How many days will Mohan alone take to complete the same task?

$5\frac{1}{4}$ days

Ravi(R) + Mohan(M) = 3 days,

Ravi(R) = 7 days,

⇒ R + M = 7,  (Efficiency)

⇒ 3 + M = 7, 

⇒ M = 4,

⇒ Time required by Mohan to complete the total work alone = \(\frac{21}{4}\) = \( {5 }_{4 }^{1 } \) days,    ..(\(\frac{Work}{Efficiency}\) = Time)