Solution: This is a typical problem of cattle eating grass.

Set up 1 set to treat 1 set of sewage 1 hour.

If two units are started at the same time, it will take 30 hours to treat sewage, and 2×30=60 sewage units will be treated within 30 hours.

If four units are started at the same time and it takes 10 hour to treat sewage, then 10 hour will treat 4× 10 = 40 units of sewage.

The amount of sewage treatment twice is different, because the time of sewage treatment twice is different.

Therefore, the sewage flowing into the storage tank from the urban area flows into (60-40)÷(30- 10)= 1 unit sewage every hour.

The reservoir contains sewage to be treated ***(4- 1)× 10=30 units of sewage (or (2- 1)×30=30 units of sewage is also acceptable).

Therefore, if sewage is required to be treated within 5 hours,

Then at least 30÷5+ 1=7 units should be started at the same time.

Hope to adopt, thank you!