Assuming that the water level rises by X meters due to the inflow of flood every hour, and drops by Y meters due to the flood discharge of each sluice 1 hour, it is known that there are

(x-y)*2=0.66

Note that two more gates have been opened, so there are three gates working in the afternoon.

(3y-x)*4=0. 1

Simultaneous solution

x=0.5075，y=0. 1775

So if five gates are opened, the water level will drop by 0. 1775 * 5-0.5075 = 0.38m per hour.

1.2/0.38=3. 15, which means it takes 3. 15 hours to reach the safe water level.

If it needs to be lowered to a safe water level within 6 hours, that is, it needs to be lowered by 0.2m within 1 hour, it needs to be opened.

(0.5075+0.2)/0. 1775=3.98

So we have to open four gates.