Решение задачи N3276 теория вероятностей

3276. In the urn there are 28 balls of the same size and appearance: among them 7 are white and 21 are black. 4 balls are taken at random (without returning to the urn). What is the probability that among them will be 3 white?

Detailed solution. Decorated in Microsoft Word 2003 (Quest decided to use the formula editor)