Solution of problem N2977 probability theory

Solution of problem N2977 probability theory

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.

Additional information:

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