Question

There are 170170 calories in a cup of whole milk and only 9797 in a cup of skim milk. In switching to skim​ milk, find the percent decrease in number of calories.

Answer 1

From 170 to 97, there's a difference of 73.

so hmm if we take 170 to be the 100%, what is 73 in percentage off of it?

$\bf \begin{array}{ccll} amount&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 170&100\\ 73&p \end{array}\cfrac{170}{73}=\cfrac{100}{p}\implies p=\cfrac{73\cdot 100}{170}$