Standard 100w light bulbs cost 40p each and are expected to last for 2 year in average use. Low power equivalent brightness bulbs cost £10 each, are expected to last 8 years and use only 20W. A 100W light bulb operated for 10 hours will use 7.5p of electricity. A 20w bulb will operate for 50 hours on the same amount of electricity. I have a light in my living room which is on for about 20 hours per week. Next time the living room bulb breaks, I will buy a low wattage build instead of a standard on.
How long will it before i have recovered the extra purchase cost?
70 weeks, 77 weeks, 80 weeks, 83 weeks or 96 weeks?
NB what is the actual purchase cost the 40p/1000p, that's whats confusing me about this calculation? Or is it the relative Watt:price ratio over a given time?
Any help appreciated!
I get exactly $80$.
Check what you did, probably due to rounding error, over which you need to be very careful, sometimes wrong answers are devilishly close to the right one !
Difference in purchase price $= 1000-40 = 960p$
Current weekly cost $= 15p$
New weekly cost $= \dfrac15\cdot15 = 3p$
Weeks needed to recover extra purchase cost $= \dfrac{960}{15-3}=80$