The average internet usage of a student for 15 days was 90 min/day. During the first 7 days, his average usage was 87 min/day and during the last 7 days, the average usage was 92 min/day. What was his internet usage on the 8^th day?

The total usage for 15 days is 15 times 90, which equals 1350 minutes. The total for the first 7 days is 7 times 87, which equals 609 minutes, and the total for the last 7 days is 7 times 92, which equals 644 minutes. The usage on the 8th day is 1350 minus the sum of 609 and 644, which is 1350 minus 1253, giving exactly 97 minutes.

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