10 hour, twice diarrhea.

To put it bluntly, it is assumed that one unit of water needs to be drained.

And drainage speed =n* overflow valve speed-upstream speed.

N means there are several flood discharge valves.

1/30 is the speed of the relief valve-the upstream speed.

110 is the speed of two flood discharge valves-upstream speed.

So the speed of an overflow valve is (110-1/30) =115.

Upstream water velocity =115-1/30 =1/30.

Speed of three relief valves-upstream speed =115 * 3-1/30 =1/6, 6 hours.

Four115 * 4-1/30 = 7/30, 30/7 = 4.3 hours.

So we have to open four.