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.