Question 791902
Suppose it will be filled in t minutes.


{{{t/20 + t/10 - t/12 = 1}}}
{{{(3 + 6 + 5)t/60 = 1}}}
{{{14t=60}}}
{{{t=60/14}}}


It will take 1/14th of an hour to fill the reserviour completely.


Cheers!