An electric bulb of 60W burns for 5 hours a day. The cost of electricity in a month of 30 days at 3 Rs per unit(Kilo watt hour-kWh ) is

Given:
Power = 60 Watt
Everyday use = 5 hours
Total Days = 30
Cost Of electricity per unit = Rs. 3
Total amount to be paid in 30 days.
The days can be calculated in total hours used in 30 days
Hence, Total hours = Days x Everyday Use$=30\times 5=150 \mathrm{hours}$
Convert, 60 Watt into Kilowatt$=\frac{60}{1000}\mathrm{kW}$
By Formula,

Energy consumed = Power(kW) x Time(hr)
$\mathrm{U}=\frac{60}{1000}\times 150$
U = 9 kWh
Hence, the total unit used is 9.
By Formula,

Cost = Unit x Rs./Unit

Cost = 9 x 3
Cost = Rs. 27
Therefore, the amount to pay in 30 days is Rs. 27.