Mathematics · Aptitude

A refrigerator has a power rating of 400 W. It is operated for 24 hours per day for the first 10 days of a 30-day month. To reduce electricity consumption, it is kept off for a fixed number of hours every day during the remaining 20 days. If the total cost of electricity consumed by the refrigerator at the end of the month is Rs. 1392 at a rate of Rs. 6 per unit (1 unit = 1 kWh), for how many hours per day was the refrigerator operated during the last 20 days?