1. Suppose there are x people, X/8 people left and 27 people came.

These 27 people have an extra 27-X/8,

There are more people in the library than at the beginning: 3* 12/2- 12=6.

That is 27-X/8=6.

X= 168 people

Now the number of people is 168+6= 174.

2.

Suppose the water level increases by x cm, then the height of the water level is X+2.5.

The added volume is the volume of cubic iron in water.

The volume of a cube in water is 6*6*(X+2.5).

The increased water quantity is: X*72.

So there is 6*6*(X+2.5)=72X.

X=2.5 cm

So the water height at this time is: 2.5+2.5 = 5cm.