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Need a Hint?

HINT: Time = Distance / Speed.

See Answer

ANSWER: 5.25 miles per hour.

EXPLANATION: Let's use x to signify the distance between Los Angeles and San Diego. Since time traveled (hours) equals distance traveled (miles) divided by average speed (miles per hour), it takes x/21 hours to travel to San Diego and then x/3 hours to return to Los Angeles. The total time on the road is therefore x/21 + x/3. To add these fractions, we need to use a common denominator of 21: x/21 + 7x/21 = 8x/21 hours. So it takes 8x/21 hours for the return trip of 2x miles or more simply 4/21 hours to travel 1 mile. Therefore, the average speed is 21/4 or 5.25 miles per hour.

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