Exercise 47 Page 290

Here is what we are told.

In $2007,$ a group of researchers found that daylight saving time decreased national energy usage by $0.2\%$ with a margin of error of $1.5\%.$ A $1\%$ decrease in energy use will save the state of Indiana $\$9$ million.

First, let's think about the change in energy usage. This usage decreased by $0.2\%$ with a margin or error of $1.5\%.$ Then, the range of the energy usage is the $\text{estimated}$ $\text{energy}$ $\text{use}$ plus or minus this $\text{margin}$ $\text{of}$ $\text{error}.$ In other words, the change in energy usage was in the range of $0.2\%-1.5\%$ and $0.2\%+1.7\%.$ Now, we know that a $1\%$ decrease in energy use, or a $\text{-}1\%$ change, saves the state of Indiana $\$9$ million. The amount of money saved is always proportional to the change in the energy use. Therefore, the change in the cost is the percent change multiplied by $\$9$ million. Let's use this fact to find the change in the cost of energy caused by the Daylight Saving Time.

Percent Change	Change in Cost $(\$)$
$\text{-}1$	$\text{-}1(9000000)=\text{-}9000000$
$\text{-}1.3$	$\text{-}1.3(9000000)=\text{-}11700000$
$+1.7$	$+1.7(9000000)=15300000$

Remember, an increase in energy use will be an additional cost for the state of Indiana, not a savings! Then, according to these results, the state of Indiana might have saved as much as $\$11.7$ million or overspent up to $\$15.3$ million because of Daylight Saving Time.