**21. Correct. The answer is true**. 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:

_{}(_{8}C_{2})(_{3}C_{1})/(_{20}C_{3}) = _{}