The lock has 4 wheels, each labelled 0–9. The first digit is known to be 7 and no digit is repeated. How many remaining 3-digit sequences must be checked?
A$720$
B$1000$
C$504$
D$343$
Answer & Solution
Correct answer: C. $504$
First digit fixed (7). Remaining 3 positions filled from the 9 unused digits with no repetition: $9 \times 8 \times 7 = 504$.
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