12. An aluminum disk with a diameter of 20 cm weighs 2.4 kg. A worker cut out a disk with a diameter of 10 cm from it. What is the weight in kilograms of the remaining part?
a) 1.2

b) 1.6

c) 1.8

d) 1.9

e) 2
12. 1.8 kg. Area of the aluminum disk: $S=10^{2} \pi=100 \pi \mathrm{cm}^{2}$, then per 1 kg, $\frac{100 \pi}{2.4}=41 \frac{2}{3} \pi \mathrm{cm}^{2}$. Area of the cut-out disk: $S=5^{2} \pi=25 \pi$ $\mathrm{cm}^{2}$. We find the mass of the cut-out part $25 \pi: 41 \frac{2}{3} \pi=0.6$ kg, so the mass of the remaining part is $2.4-0.6=1.8$ kg.