The man exetrs a force F (say) so that his effective wieght is (mg - F) as shown by the weighing machine.(m = mass of the man)



weight of the box + effective wieght of man = force applied by man

30g + 60g - F= F

or F=45kg



thus the weight shown by the machine = mg - F=15 kg

now if u apply force F reading is 15kg. To get correct wiehtg (60 kg) we must apply 4F=4*45kg=180kg=1800N.