Heat taken by water when its temperature changes from 20°C to 100°C. H1=mc(θ2−θ1)=1000×1×(100−20)‌cal =1000×80×4.2‌‌J Heat produced in time t due to current in resistor H2=V×i×t=220×4×t‌‌J According to question, 220×4×t=1000×80×4.2 ⇒ t=