SOLUTION: If Machine A makes a yo-yo every five minutes and Machine B takes ten minutes to make a yo-yo, how many hours would it take them working together to make 20yo−yos?

We assume that both machines started simultaneously.


Every 10 minutes, machine A produces 2 yo-yo; every 10 minutes machine B produces 1 yo-yo.

So, two machines produce 3 yo-yo every 10 minutes.


Divide 20 by 3 with a remainder: 20/3 = 6 + 2/3.

It tells you that in 6*10 = 60 minutes two machines will produce 6*3 = 18 yo-yos,
which is still less than 20 yo-yous.


In the next 5 minutes, machine A will produce the 19-th yo-yo.
In the next 5 minutes, machine A will produce the 20-th yo-yo, and machine B will complete the 21-th yo-yo.


Thus 20 yo-yo (and even 21 (!) yo-yo) will be completed 70 minutes after start moment.


70 minutes is the minimal time to wait to have 20 yo-yos completed.