Question 436132
<pre>
               rate         time        amount

filling        1/60         60           1

draining       1/80         80           1

combined    1/60-1/80        x           1
</pre>
The rates are additive, but need to understand draining is negative while filling is positive.

{{{1/60-1/80}}} ponds per minute

{{{4/240-3/240}}}

{{{1/240}}}

That is 1 pond in 240 minutes, or 240 minutes to fill the 1 pond with both flows opened.