Решение задачи N2976 теория вероятностей

2976. The first box contains 20 parts, 15 of which are standard; in the second - 30 parts, of which 24 are standard; in the third - 10 parts, of which 6 are standard. Find the probability that at random the extracted part from the randomly taken box is standard.

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