A particular substance is cooled by a stream of cold air at a constant temperature of 5∘C. The rate of cooling is directly proportional to the square of the difference between the temperatures of the substance and air. If the substance is cooled from 40∘C to 30∘C in 15 min and its temperature after 1 hour is t∘C, find 2[t], where [⋅] represents the greatest integer function.