Answer: it will take them 48 minutes.
Explanation:
John can mow a lawn in 80 minutes. This means that the rate at which he moans the lawn per minute is 1/80
Rocky can mow the same lawn in 120 minutes. This means that the rate at which Rocky can mow the same lawn per minute is 1/120
If they work together, they would work simultaneously and their individual rates are additive. This means that their combined working rate would be
1/80 + 1/120 = 200/9600 = 1/48
Assuming it takes t hours for both of them to clean the room working together, the working rate per hour would be 1/t. Therefore,
1/48 = 1/t
t = 48 minutes