A machine dissipates 300 J for every 700 J transferred usefully. It transfers 4200 J usefully in 12 s. What is its mean input power?
350 W350\ \mathrm{W}350 W
150 W150\ \mathrm{W}150 W
700 W700\ \mathrm{W}700 W
500 W500\ \mathrm{W}500 W