Solution: Assume that the incoming water is V 1, the outlet water of each sluice is -V2, and the existing water level exceeds the safety line as A.
According to the meaning of the question:
a+30(v 1-V2)= 0 & gt; & gt30V2-30V 1=A ①
a+ 10(v 1-2 v2)= 0 & gt; & gt20V2- 10V 1=A
That is, 30V2-30V1= 20V2-10V1>; & gt 10 v2 = 20v 1 & gt; & gtV2=2V 1 substituted into ①.
a+30v 1-30 v2 = 0 & gt; & gta+30v 1-60v 1 & gt; & gtA=30V 1= 15V2
2.5 hours to the safety line, you need to open n doors.
2.5(V 1-NV2)=A
2.5v 1-5nv 1 = 30v 1
5N=32.5
N=32.5/5=6.5≈7
Therefore, it is necessary to open seven flood discharge tunnels.
I hope the answer is useful to you.