Answer: it will take both of them 2.4 hours to complete the job.
Explanation:
Person A can complete a job in 6 hours. This means that the rate at which Person A can complete the job per hour is 1/6
Person B can complete the same job in 4 hours. This means that the rate at which Person A can complete the job per hour is 1/4
If they work together, they would work simultaneously and their individual rates are additive. This means that their combined working rate would be
1/6 + 1/4 = 5/12
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,
5/12 = 1/t
t = 12/5
t = 2.4 hours