Algebra I in Simple English/Proportions and Proportional Reasoning/Percents
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[edit] Vocabulary
Percent: Parts per one hundred
[edit] Lesson
Suppose we had a grid of 100 squares, of which 17 are shaded in. There are several ways to express this as a ratio, as you learned in the previous lesson, such as 
, the number of shaded squares compared to the number of squares in total. However, we can also write this as a percent. Since there are 17 shaded squares and 100 total, we say that 
of the squares are shaded. The 
symbol stands for "percent".
Now suppose that instead of 100 squares, we have 50 squares, with 9 of them shaded. The percent shaded would not be 
, because percent means "per 100", and we have 9 shaded squares out of 50 total squares. To find the percent, we need some number over 100, so we can set up a proportion.
Cross-multiplying gives us 
, and x = 18.
We also could have noticed that 
, so we just need to multiply the numerator, 9, by 2 to get our answer, 18.
