Решение задачи N2977 теория вероятностей

2977. The first box contains 20 parts, of which 12 are standard; in the second - 30, of which 16 are standard; in the third - 10 parts, of which 6 are standard. Find the probability that the extracted part from the selected box will be standard at random.

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