Company A can set up the gymnasium for a banquet in 3 hours. Company B needs 5 hrs to do the same job. How long would it take them working together, to set up the gymnasium?
For problems like this, you need to consider how much work each individual (company in this case) can do in one hour.
Hence, when company A and B work together, the gymnasium will be set up in 1 hour, 52 minutes and 5 seconds.