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.

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