CBSE Class 10 Science 2018 · Paper 31/1/C · Question 13

Energy used in September ($30$ days), taking $1$ unit $=1$ kWh: (i) Electric heater: $E_1=1\text{kW}\times 5\text{h}\times 30=150$ kWh $=150$ units. (ii) Electric refrigerator: $E_2=0.4\text{kW}\times 10\text{h}\times 30=120$ kWh $=120$ units. Total energy $=150+120=270$ units. Cost in rupees $=270\times 6.00=1620$. Total cost of running the two devices in September = ₹1620 (heater ₹900, refrigerator ₹720).