Question

A company has two large computers. The slower computer can send all the company's email in 35 minutes. The faster computer can complete the same job in 15 minutes. If both computers are working together, how long will it take them to do the job

Answer 1

The rate for the first is 1 job / 35 minutes and for the second is 1 job / 15 minutes.  So combined we get

r = 1/35 + 1/15

3×5×7r = 3 + 7 = 10

r = 10/(105) jobs per minute

We're interested in 1/r

1/r = 105/10 = 10.5 minutes per job

Answer: 10.5 minutes