SAT Math Multiple Choice Practice Question 80: Answer and Explanation

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Question: 80

14. How many two-digit numbers contain one even digit and one odd digit?

A. 20
B. 25
C. 45
D. 50
E. 55

Correct Answer: C

Explanation:

C. First, realize that this problem has two possibilities: an even number followed by an odd, or an odd followed by an even. If the first number is even, it could be 2, 4, 6, or 8 (four possibilities). The second number must then be 1, 3, 5, 7, or 9 (five possibilities). Using the counting principle, 4 × 5 = 20 possible numbers. If the first number is odd, it could be 1, 3, 5, 7, or 9 (five possibilities). The second number must then be 0, 2, 4, 6, or 8 (five possibilities); don't forget about zero! So you have 5 × 5 = 25 more possibilities, for a total of 20 + 25 = 45 possible two-digit numbers.

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