We are given the ratio of 1/2 inch: 18 miles
Multiply both sides of the ratio by 2. I'm doing this so that the "1/2" turns into "1"
So we go from this ratio
1/2 inch: 18 miles
to this ratio
1 inch: 36 miles
which means that every inch on paper represents 36 miles in real life
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The question is now: if two towns are 2.5 inches apart on paper, then how far are they in real life? (note: 2.5 is the decimal form of 2 and a half)
To find out, multiply both sides by 2.5
1 inch: 36 miles
2.5*(1 inch): 2.5*(36 miles)
2.5 inches: 90 miles
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Answer: The two towns are 90 miles apart in real life