**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:

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