21. Incorrect. The answer is true not false. We can calculate the probability by counting: a) the number of ways in which 2 first two samples can be chosen from the total of 8 samples of copper, b) the number of ways in which the last sample can be chosen from a total of 3 samples of zinc, and c) the number of possible cases. Multiplying a) and b) and dividing the product by c), we can calculate the probability. To count, we can use the combinatorial formula:




Where "n" is the total number of samples of certain mineral, "r" is the number of mineral drawn in the sample process. When we work with the combinatorial number, we are not interested in the order of arrangement of the samples. "!" is the symbol of the factorial operation which states that: n! = n(n-1)(n-2) . . 1. The probability of the event is:


(8C2)(3C1)/(20C3) =