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he would arrive 0.2 miles faster.
Distance = speed × time
Time = distance/speed
William cycles at a sped of 15 miles per hour. He cycles 12 miles from home to school. This means that the time it takes William to get to school from home would be
12/15 = 0.8 hours
If he increases his cycling speed by 5 miles per hour, his new speed becomes
15 + 5 = 20 miles per hour
Therefore, the new time it takes William to get to school from home would be
12/20 = 0.6 hours
The difference in both times is
0.8 - 0.6 = 0.2 hours
Therefore, he would arrive 0.2 miles faster.