Question 742195
<pre>
                   RATE          TIME        TANKS

ONE PIPE             1/20           20         1

ANOTHER PIPE         1/30           30         1
</pre>

If the two pipes are used together for filling the one tank, then
{{{(1/20+1/30)*x=1}}}


{{{(3+2)x=60}}}


{{{5x=60}}}


{{{x=12}}}---------------twelve minutes