Max has a small pool in the garden. He would love to jump in right away, but first he has to fill it up. There are three different pipes with different thicknesses for this, so it takes different amounts of time to fill the pool. If Max uses pipe A, it takes three hours to fill the pool. Using pipe B it takes 4 hours and using pipe C requires 6 hours. How many minutes does it take to fill the pool if Max uses all three pipes at the same time?
It takes 80 minutes to fill the pool. Per hour, pipe A fills 1/3, pipe B fills 1/4 and pipe C fills 1/6 of the pool. So together they fill 4/12 + 3/12 + 2/12 = 9/12 = 3/4 of the pool in one hour. For the last quarter, another 1/3 hour is required.