(a) Power of bulb is 100 W, converting this into kW.
![1 W=\frac{1}{1000}kW](https://tex.z-dn.net/?f=1%20W%3D%5Cfrac%7B1%7D%7B1000%7DkW)
Thus,
![100 W =\frac{100}{1000}kW=0.1 kW](https://tex.z-dn.net/?f=100%20W%20%3D%5Cfrac%7B100%7D%7B1000%7DkW%3D0.1%20kW)
The bulb is used for 6 hours per day for a year, in 1 year there are 365 days thus, total hours will be:
![t=6\times 365=2190 h](https://tex.z-dn.net/?f=t%3D6%5Ctimes%20365%3D2190%20h)
Electricity used will depend on power and number of hours as follows:
![E=P\times t=0.1 kW\times 2190 h=219 kW.h](https://tex.z-dn.net/?f=E%3DP%5Ctimes%20t%3D0.1%20kW%5Ctimes%202190%20h%3D219%20kW.h)
The cost of electricity is $0.15 per kW.h thus, cost of electricity for 219 kW.h will be:
![Cost=\$ (219\times 0.15)=\$ 32.85](https://tex.z-dn.net/?f=Cost%3D%5C%24%20%28219%5Ctimes%200.15%29%3D%5C%24%2032.85)
Therefore, annual cost of incandescent light bulb is ![\$ 32.85](https://tex.z-dn.net/?f=%5C%24%2032.85)
(b) Power of bulb is 25 W, converting this into kW.
![1 W=\frac{1}{1000}kW](https://tex.z-dn.net/?f=1%20W%3D%5Cfrac%7B1%7D%7B1000%7DkW)
Thus,
![100 W =\frac{25}{1000}kW=0.025 kW](https://tex.z-dn.net/?f=100%20W%20%3D%5Cfrac%7B25%7D%7B1000%7DkW%3D0.025%20kW)
The bulb is used for 6 hours per day for a year, in 1 year there are 365 days thus, total hours will be:
![t=6\times 365=2190 h](https://tex.z-dn.net/?f=t%3D6%5Ctimes%20365%3D2190%20h)
Electricity used will depend on power and number of hours as follows:
![E=P\times t=0.025 kW\times 2190 h=54.75 kW.h](https://tex.z-dn.net/?f=E%3DP%5Ctimes%20t%3D0.025%20kW%5Ctimes%202190%20h%3D54.75%20kW.h)
The cost of electricity is $0.15 per kW.h thus, cost of electricity for 54.75 kW.h will be:
![Cost=\$ (54.75\times 0.15)=\$ 8.21](https://tex.z-dn.net/?f=Cost%3D%5C%24%20%2854.75%5Ctimes%200.15%29%3D%5C%24%208.21)
Therefore, annual cost of fluorescent bulb is
.