1995 AMC 8 Problem 23

Attempt Problem 23 of the 1995 AMC 8 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1995 AMC 8 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

23.

How many four-digit whole numbers are there such that the leftmost digit is odd, the second digit is even, and all four digits are different?
11201120
14001400
18001800
20252025
25002500
Answer: B
Concepts:multiplication principle
Small hint:
Count choices for each digit position in turn: first, second, third, fourth
Big hint:
First digit: 55 odd options; second: 55 even options; then 88 and 77 for the distinct remaining digits
Solution:
The first digit is odd: 55 choices. The second is even: 55 choices (none of which repeats the odd first digit).
The third digit is any of the 88 unused digits, and the fourth is any of the 77 remaining. In total, 5×5×8×7=1400.5 \times 5 \times 8 \times 7 = 1400.
Thus, the correct answer is B .
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