Решение задачи N5122 теория вероятностей

5122. In a batch of 30 identically packaged articles, articles of two grades are mixed, with 20 of these articles being of the first class, and the rest of the articles of the second class. Find the probability that two products taken at random will be: a) of the same sort; b) different varieties.

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