a)
[tex]\dfrac{\$0.10}{1000\,Wh}\times 50\,W\times 9\,h=\dfrac{\$0.10\cdot 50\cdot 9}{1000}=\bf{\$0.045}[/tex]
b) The cost per year is 365 times this amount. You save 35/50 times that, so you save ...
... $0.045×365×35/50 = $11.4975 ≈ $11.50 . . . annual savings
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If the bulb is using 15 W instead of 50 W, you are saving the cost of 50-15=35 W, so 35/50 of the cost of running the higher-power bulb.
