(2) Current through the coil I=P/U=38×10^3W/380V=100A.
Heat emitted by the coil Q=I^2Rt=(100A)^2×0.4Ω×20s=8×10^4J.
(3) Useful work done on the suspended weight W`=W×80%=6.08×10^5J.
Gravitational force of the weight G=F=W`/s=6.08×10^5J/20m=3.04×10^4N.
Mass of the weight m=G/g=3.04×10^4N/(10N/kg)=3.04×10^3kg =3.04t. (Take g=10N/kg)