Ajutoooooooooooooooooooooor

t=40 min=2400 s
Qc=mcΔθ=2*4190(100-20)=8380*80=670400 J
R=U/I=120/2,5=48 Ω
Qu=RI²t=48*2,5²*2400=720000 J
η=Qc/Qu=670400/720000=0,931