Question

After each charging, a battery is able to hold only 96% of the charge from the previous charging. The battery was used for 10 hours on its first charge before it had to be recharged. What is the total number of hours the battery can be used over its lifetime?

Answer 1

250 hours

Explanation:

The first charge is 10 hours, the second charge is 10(0.96) hours, and so on -- the total of these two is 10+10(0.96) and we need to keep adding our battery usage up until there is no more battery. The third one is 10(0.96)*(0.96), or 10(0.96)².

This can be seen as a geometric series, with 10 as the first tern, or a, and 0.96 as the common ratio, or r. The sum of this is

`a(1-r^(n))/(1-r)`, with n being the number of terms. Since 0.96 to the power of n never actually reaches 0 no matter how high n goes if the number is not infinite, we can say n is infinity. As r^n is really close to 0 when n equals infinity (so close that, for our purposes in this equation, we can just assume it's 0), our equation is then

`10(1)/(0.04) =10*25=250`