Question 652555
<pre>
20 minutes per hours means that each pump is running {{{20/60}}} or 
{{{1/3}}} of the time

So at any time, the probability that any pump is running is {{{1/3}}}

P(1st running AND 2nd running AND 3rd running) = 

P(1st running)×P(2nd running)×P(3rd running) = {{{1/3}}}×{{{1/3}}}×{{{1/3}}} = {{{1/27}}} 

Edwin</pre>