In the past, the mean height of plants of a particular species has been 2.3m. A random sample of 60 plants of this species was treated with fertiliser and the mean height of these 60 plants was found to be 2.4m. Assume that the standard deviation of the heights of plants treated with fertiliser is 0.4m. Carry out a test at the 2.5% significance level of whether the mean height of plants treated with fertiliser is greater than 2.3m. [5]