Irwin rode his bicycle from one end of a trail to another and back at a speed of 20 miles per hour. If the trail is 4 miles long, for how many hours was Irwin riding?

Irwin rode 4 miles to the other end and 4 miles back, for a total of 8 miles. At 20 miles per hour, the time was 8 ÷ 20 = 0.4 hours, so the correct choice is 0.4 hours.

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