In decimal form, to the nearest tenth, what is the probability that a randomly selected Riverside High School student is in twel
fth grade?
2 answers:
We have that the total students there are 500. The 12-graders there are 200. Probability is defined as the ratio of positive outcomes of an event, over all the possible outcomes. Suppose we pick student randomly. Then, there are 200 positive outcomes (positive outcome: we pick a student in 12th grade) and there are totally 500 outcomes (we can pick 500 students in total from Riverside High School). This ratio gives: 
. The requested probability is 0.40
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and is all you can simplify from it.
so... all we did, was rationaliize it, namely, "getting rid of the pesky radical at the bottom", we do so by simply multiplying it by something that will raise the radicand, to the same degree as the root, thus the radicand comes out.