The answer is: [A]:  52.5 (mi.) 
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Explanation:
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Use the formula: D = r*t .
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{Note of interest: The formula is not unique to this specific problem; but is actually true and will appear in physics and even advanced math and calculus!).
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In which: D = distance traveled (which we want to solve);
                  r = rate of travel; which we are given as: “15 mi / h” ;
                  t  = time traveled; which we are given as: “3.5 h” ,
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We want to solve for “D”.
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 Make sure all the units are equal (which they are!). They are all in “mi.” and “hours”.  And the questions asks for the value in miles
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D = r * t ; solve for “D” ; by plugging in our known values of “r” and “t”:
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→ D = r * t  =  frac{15 mi}{h} * (3.5 h) ;  
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→ The 2 “h’s” cancel out; and we are left with:
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→ D = (15 mi.) * 3.5 = 52.5 mi. → which is answer choice: [A].
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