Solution of problem N3561 probability theory

Solution of problem N3561 probability theory

3561. In a basket 7 balls with numbers 1,2,3,4,5,6,7. Randomly choose three balls, without return. Find the probability that: a) consistently there will be balls with numbers 2,6,7 b) the extracted balls will have numbers 2,6,7 regardless of the sequence in which they appeared.

Additional information:

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