Here we have two physical quantities which we have to divide
other is
now we need to find its division
now we have
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Initially there were 10 bulbs of 60 Watt power
So total power of all bulbs = 60 * 10 = 600 W
now each bulb used for 4 hours daily
so total energy consumed daily
now we have total power consumed in 1 year
cost of electricity = 10 cents/ kWh
so total cost of energy for one year
Now if all 60 Watt bulbs are replaced by 30 Watt bulbs
So total power of all bulbs = 30 * 10 = 300 W
now each bulb used for 4 hours daily
so total energy consumed daily
now we have total power consumed in 1 year
cost of electricity = 10 cents/ kWh
so total cost of energy for one year
total money saved in 1 year