To determine the total class the whole class spent, you just have to simply add the individual costs of the three books. Using parenthesis, the solution can be written in the following two ways:
($15 + $9) + $12 = $36
$15 + ($9 + $12) = $36
No matter where you put the parenthesis, the final answer doesn't change. This is because of the Commutative Property of Addition. The order does not matter.
It is an irrational number. That is because rational numbers can be put into fractions. But because this decimal goes on forever, you can't put it into a fraction. This is also not a whole number or an integer. Whole numbers and integers do not have any decimals or fractions in them, but this number is a decimal. Therefore, this number is irrational.
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