If one person can do a job in 60 minutes and another per can do it in 90 minutes, how long will it take them, working together, to do the job?
first person does 1/60 job/min
second does 1/90 job/min
together they do
1/60 + 1/90 = 3/180 + 2/180 = 5/180 job/min
so
(5/180)t = 1 job
t = 180/5 = 36 minutes