In: Statistics and Probability
For each question below, calculate the number of four-digit integers (1000 through 9999 inclusive; the first digit cannot be 0) that satisfy the specified property:
1) How many four-digit integers are even (for example, 2102 and 8162, but NOT 2001)?
2) How many four-digit integers are consisted of four distinct digits in strictly decreasing order (for example, 9621 and 8742, but NOT 1352)? (Hint: combination problem)
3) How many four-digit integers are consisted of two pairs of distinct digits (for example, 1001 and 2424, but NOT 3333)?