Xavier exercises by riding his bike. During his first week of training, he rides a total of 15 miles. Each week he increases the number of miles biked by 0.4 miles. How many total weeks will it take Xavier to first bike more than 500 miles total?

The weekly distances form an arithmetic sequence: 15, 15.4, 15.8, and so on. After n weeks, the total is n/2 × [15 + (15 + 0.4(n − 1))]. After 25 weeks, the total is 495 miles, which is not more than 500; after 26 weeks, it is 520 miles. So Xavier first exceeds 500 miles in 26 weeks, choice D.

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