2850. In a group of 30 students in the test, 6 students received "5", 10 students - "4", 9 students - "3". Find the probability that at least 1 student out of 3 called to the board has a grade of a) 5 and 4; b) 3.

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