Решение задачи N2859 теория вероятностей

2859. In a group of 30 students in the control work on the "Theory of Probability" 6 students were rated "excellent", 10 - "good", 9 - "satisfactory", the rest - "unsatisfactory." Find the probabilities that: a) all three students called to the blackboard have an “unsatisfactory” grade for the test; b) at least one of the three students called to the board has an “excellent” or “good” rating.

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