Question

Suppose a family replaces ten 60-watt incandescent bulbs with ten 30-watt

Answer 1

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

E = p*t

E = 600*4 = 2400Wh

E = 2.4kWH

now we have total power consumed in 1 year

E = 365 * 2.4 = 876kWh

cost of electricity = 10 cents/ kWh

so total cost of energy for one year

P1  = 876*10 = 8760cents = 87.60

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

(The next steps are in the pic.)