Solution of problem N4972 probability theory

Solution of problem N4972 probability theory

4972. There are 26 parts in the box, among which 21 are standard. The buyer is sent 2 parts. Find the probability that:
1) all non-standard parts were left in the box;
2) among the sent parts, one part is standard and the other is non-standard;
3) at least one of the two sent parts will be standard.

Additional information:

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