Answer:
31.5 hours
Step-by-step explanation:
Given that 1 unit of electricity is consumed in 3 hours by 1 fan, it means that 2 fans will consume 1 unit in
= 3/2 hours
= 1.5 hours
If 2 fans consume 1 unit in 1.5 hours then 21 units will be consumed in
= 21 * 1.5/1
= 31.5 hours
It means he used the fans for 31.5 hours
