Use the multiplication property of equality. It tells you that multiplying both sides of an equation by the same value does not change the values of any of the variables. Here we can multiply by 12 and we get ...
r(12/12) = 1.5(12)
Now, the properties of the identity element for multiplication come into play. When we multiply r by 12/12=1, we do not change the value of r, so we can simplify this to ...
r = 18
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We chose to multiply by 12 precisely because 12/12 = 1, and the result on the left side of the equation is r.
