Question:
<p>Mr. A forgot a phone number. He remembers it starts with 713 and the remaining 4 digits are some arrangement of 1, 7, 5, 9 (each used exactly once). He dials a number at random. The probability that he dials the correct number is</p>
Step-by-Step Solution
Key Concept: The 4 unknown digits are an arrangement of {1,7,5,9}. Total arrangements = 4! = 24.
<p>The first three digits (713) are known. The remaining 4 digits are a permutation of $\{1, 7, 5, 9\}$.</p><p>Total arrangements $= 4! = 24$.</p><p>$P(\text{correct}) = \dfrac{1}{24}$</p>
Correct Answer: B