Problem Solving

Each of 10 machines works at a constant rate to do a certain job. The amount of time needed by the 10 machines working together is 16 hours.

How many hours would be needed if only 8 machines, working together, were used to complete the job?

Answer is 18

you sure the answer is 18?

16 hours * 10 machines = 1 unit

x hours * 8 machines = 1 unit

so 16*10 = 8x

x=20

OR

Let's say the job consisted of 80 units.

This means that each machine can do 80/10 units in 16 hours.
Or 8/16 units/hour/machine = .5 units per hour.

We now want to do 80 units with 8 machines.

So 8 * .5 = 4 units/h (with 8 machines)

80 units / 4 units/h = 20h

ok.. just saw you posted this twice...

https://www.beatthegmat.com/rate-problem ... 12131.html

Thanks so much!!