As we have a total of 23 students and we want to form groups of 3, where the order of the 3 students matters, we can solve this problem using a permutation of 23 choose 3.
So, for a small difference in rainfall of 220 mm, what is the corresponding small difference in width of leaves in the two forests given.
One definition of a derivative or a rate of change is that it is the ratio of very small differences in the dependent variable to very small differences in the independent variable.