A particular substance is being cooled by a stream of cold air (temperature of the air is constant and is 5∘C ) where rate of cooling is directly proportional to square of difference of temperature of the substance and the air. If the substance is cooled from 40∘C to 30∘C in 15 min and temperature after 1 hour is T∘C , then find the value of [T]/2 where [⋅] represents the greatest integer function.