Well, you have a good starting point. You have already figured out that $23.50 is the amount of money that it will cost to use a light bulb for 10 years, assuming there is no inflation.
So, divide that number by 10 to get the amount of money it will cost for 1 year of use.
Now that you have the amount in one year, $2.35, you can use the compound interest formula to find the amount of money used during the other years and simply add them together.
Total Cost .....
There is a way to represent this using this , but I am not sure how to use LaTex to do that, so I will let you find this yourself.