f = G = mg = 150× 10-3kg× 10N/kg = 1.5N，

Pressure of glass on desktop:

p=FS= 1.5N80× 10？ 4m 2 = 187.5 pa；

(2) Water filling quality:

M water = 150g-70g=80g,

∑ρ= mV，

∴ Volume filled with water:

V water =m water ρ water =80g 1g/cm3=80cm3,

Glass volume:

V=V water =80cm3,

Water quality when plexiglass is put into the cup and refilled with water;

M water = 154g-70g-9g=75g,

Volume of water:

V water' =m water ρ water =75g 1g/cm3=75cm3,

The volume of a glass:

V resin glass =V-V water =80cm3-75cm3=5cm3,

Density of plexiglass:

ρ=m mirror v mirror = 9g5cm 3 =1.8g/cm3 =1.8×103kg/m3.

So the answers are: 187.5, 1.8 × 103.