One machine can complete a job in 10 minutes. If the same job is done by this machine and an older machine working together, the job can be completed in 6 minutes. How long would it take the older machine to do the job alone?

15 minutes

Step-by-step explanation:

The speed of the first machine is 1/10 job/minute.

The speed of the older machine is x.

Their combined speed is:

x + 1/10 = 1/6

Solving for x:

x = 1/6 - 1/10

x = 5/30 - 3/30

x = 2/30

x = 1/15

The speed of the older machine is 1/15 job/minute, so it takes 15 minutes for the older machine to do the job alone.