Meera, Nina and Seema can complete a piece of work in 9 days, 12 days and 18 days respectively. If they start with Nina, then Meera and then Seema work alternately every third day, then in how many days will the work be completed?

Correct option is D

Let the total work is 36 Units (Lcm of 9, 12, 18)
Efficiency = Total work/ Total time
A's efficiency = 36/9 = 4
​B's efficiency = 36/12 = 3
C's efficiency = 36/18 = 2

=> A, B and C does work in three days = 4 + 3 + 2 = 9 units
=> Time taken to do 36 units = 36/9 = 4
But 9 Units done in 3 days
So, Actual time is 4 x 3 = 12 days