Mathematics - Discrete Mathematics - Combinatorics - Counting Problems

10 problems: (1) 'How many 4-digit numbers have no repeated digits?' (9×9×8×7). (2) 'How many ways to distribute 8 identical balls into 4 boxes?' (Stars and bars: C(11,3)). (3) 'In a group of 100, at least how many share a birthday month?' (⌈100/12⌉=9). (4) 'Expand (2x+3)⁴ using binomial theorem.' (