If Machine P finishes a job in 5 hrs, and a Machine Q finishes 2 jobs in 15 hours, how long does it take them, working together, to finish one job? (Assume all "jobs" are the same size)
Always add rates!
Convert each of those times to a job-per-time rate.
Rate A = 1/5
Rate B = 2/15
Combined = 1/5 + 2/15 = 3/15 + 2/15 = 5/15 = 1/3
The combined rate is 1 job in 3 hours, so it takes them 3 hours for 1 job.