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 point 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 miles in week 1, increasing by 0.4 mile each week. By week 26, the distance for that week is 15 + 25(0.4) = 25 miles, so the total is 26(15 + 25)/2 = 520 miles. By week 25, the total is 25(15 + 24.6)/2 = 495 miles, so Xavier first exceeds 500 miles in week 26, choice D.

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