Question 1202478
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The distances to the duck pond and back are the same, so the ratio of times at the two speeds is the opposite of the ratio of speeds.<br>
The ratio of the two speeds was 3:7, so the ratio of times was 7:3.  Since the total time was 10 hours, simple reasoning says they spent 7 hours going and 3 hours returning.<br>
7 hours at 3 mph means the distance was 21 miles; likewise, 3 hours at 7 mph means the distance was 21 miles.<br>
ANSWER: 21 miles<br>