Given
Cesar can paint a room in 15 hours Alan can paint the same room in 12 hours,
To find: How long does it take for both Cesar and Alan to paint the room if they are working together.
Step-by-step explanation:
It is given that,
Cesar can paint a room in 15 hours Alan can paint the same room in 12 hours,
Then, the work done by Cesar is 1/15, and the work done by Alan is 1/12.
Therefore, the total work done by Cesar and Alan is,

Hence, the Alan and Cesar took 20 hours to paint the room together.