Crystal rode her bike 600 miles and she carried a spare tire on the trip. To provide equal wear on all tires she changed her tires while traveling. How many miles of wear did each tire accumulate?

• A. 200 miles
• B. 400 miles
• C. 300 miles
• D. 600 miles

Answer: (B)

Two tires are in contact with the road at any moment, while a spare is carried along. To keep wear even, you rotate so each tire ends up doing an equal share of the miles.

Over the whole trip, wear accumulates for two tires for every mile traveled, so total tire wear is 2 × 600 = 1200 tire-miles. If that wear is spread equally among three tires, each tire would accumulate 1200 ÷ 3 = 400 miles of wear.

So each tire wears 400 miles.