On a given workday, the rate, in tons per hour, at which unprocessed ore arrives at a refinery is modeled by O(t) = 20 + 10 cos (t^2/25), where t is measured in hours and 0 ≤ t ≤ 10. At the beginning of the workday, t = 0, the refinery has 100 tons of unprocessed ore. During the hours of operation, the refinery processes ore at a constant rate of 25 tons per hour. Determine the time at which the total rate of ore at the refinery is neither increasing nor decreasing? Justify your answer.