The answer is 31.275 miles/hour.
Take 13.9 meters/second. Multiply that by 60 seconds/minute and multiply it again by 60 minutes/hour. Then divide that by 1600 meters/mile.
13.9 x 60 x 60 / 1600
Take 13.9 meters/second. Multiply that by 60 seconds/minute and multiply it again by 60 minutes/hour. Then divide that by 1600 meters/mile.
13.9 x 60 x 60 / 1600
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13.9 (meters / second) = 31.0934145 mph