Correct Answer - Option 4 : 11.25 hours
Given:
M can eat some scoops of ice-cream in 9 hours.
M and N together ate the ice-cream in 5 hours.
Concepts used:
1 hour work done by individual = Work/time
Calculation:
1 hour work done by individual = Work/time
⇒ 1 hour work of M = 1/9
M and N together ate the ice-cream in 5 hours.
Let N alone can eat the ice-cream scoops in y hours.
⇒ 1 hour work of N = 1/y
⇒ (1/9) + (1/y) = 1/5
⇒ 1/y = (1/5) – (1/9)
⇒ 1/y = 4/45
⇒ y = 45/4 hours = 11.25 hours
∴ N alone can eat scoops of ice-cream in 11.25 hours.