At his first speed, Kevin ran 10 miles per hour * 0.5 hours = <<10*0.5=5>>5 miles.
At his second speed, he ran 20 miles per hour * 0.5 hours = <<20*0.5=10>>10 miles.
15 minutes is a quarter of an hour so at his third speed, he ran 8 miles per hour * 0.25 hours = <<8*0.25=2>>2 miles.
Therefore, Kevin ran a total of 5 miles + 10 miles + 2 miles = <<5+10+2=17>>17 miles.
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