Pramod can paint a wall red in 12 hours while Brajen can whitewash the wall completely in 16 hours. If Pramod and Brajen work alternately for an hour each starting when the wall has just cement on it till when it is completely painted red, how many hours will it take to paint the entire wall red?

Let the total work be 48 units.
Efficiency of Pramod = 48/12 = 4 units per hour
Efficiency of Brajen = 48/(-16) = -3 units per hour
They work on alternate hours starting with empty wall.
Then, 1 unit of work will be completed in 2 hours.
44 units of work will be completed in 88 hours.
Remaining work = 4 units
4 units will be completed by Pramod in 1 hour.
Therefore, Total work will be completed in 88+1 = 89 hours.