a) What is the difference between an r-combination and an r-permutation of a set with n elements?
b) Derive an equation that relates the number of r-combinations and the number of r-permutations of a set with n elements.
c) How many ways are there to select six students from a class of 25 to serve on a committee?
d) How many ways are there to select six students from a class of 25 to hold six different executive positions on a committee?