42. •• A 200-W computer power supply is on 10 h per day.

(a) If the cost of electricity is $0.15/kWh, what is the energy cost if this computer is used like this every day for a year (365 days)?

(b) If this power supply is replaced by a more efficient 100-W version and it costs

$75, how long will it take the decreased operating cost to pay for the new supply?