Posted by Jen on .
You can buy an energysaving light bulb for $18.07, which can work for 10,000 hours. It costs you 0.8 cents to operate it for an hour. ($1=100 cents). A regular light bulb with the same intensity costs 39 cents. The light bulb can work for 1000 hours. It costs you 3.4 cents to operate it for an hour. After how many hours of operation is the energysaving light bulb more economical?
Thank You!

Math 
drwls,
Bulb A total cost (including depreciation) = 18.07 t/10,000 + 0.008 t
= 0.02607 per hour
Bulb B total cost (including depreciation) = 0.39 t/1000 + .034 t
= 0.03439 per hour
A is always less expensive, in the long run, when bulb replacement costs are included. But they probably want you to know "How soon before you save enough to pay for the energysaving bulb?". For that answer, solve this equation:
18.07 + 0.008 t = 0.39 + .034 t
17.68 = 0.026 t
t = 680 hours, before the lamp B burns out
Nowadays, compact fluorescent energysaving bulbs cost quite a bit less than 18.07, and the savings come much sooner. 
Math 
Jen,
Thank you very much! Great answer!