Samir begins riding his bike at a rate of 6 miles per hour. Twelve minutes later, Chris leaves from the same point and bikes along the same route at 9 miles per hour. How long after Chris begins riding does he catch up to Samir?

Samir has a 12-minute head start, which is 0.2 hours. At 6 miles per hour, he rides 6 × 0.2 = 1.2 miles before Chris starts. Chris gains on him at 9 − 6 = 3 miles per hour, so catching up takes 1.2 ÷ 3 = 0.4 hours, or 24 minutes. The correct choice is C, 24 min.

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