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A passcode to enter a building is a sequence of 4 single digit numbers 
(0-9), and repeated digits aren't allowed.
Suppose someone doesn't know the passcode and randomly guesses a sequence of 4 digits.
What is the probability that they guess the correct sequence?
Choose 1 answer:
(A) 
(1)/(_(10)P_(4))
(B) 
(1)/(_(10)C_(4))
(C) 
(_(4)P_(4))/(_(10)P_(4))
(D) 
((_(4)P_(2))*(_(4)P_(2)))/(_(10)P_(4))

A passcode to enter a building is a sequence of 44 single digit numbers (09) (0-9) , and repeated digits aren't allowed.\newlineSuppose someone doesn't know the passcode and randomly guesses a sequence of 44 digits.\newlineWhat is the probability that they guess the correct sequence?\newlineChoose 11 answer:\newline(A) 110P4 \frac{1}{{ }_{10} \mathrm{P}_{4}} \newline(B) 110C4 \frac{1}{{ }_{10} \mathrm{C}_{4}} \newline(C) 4P410P4 \frac{{ }_{4} \mathrm{P}_{4}}{{ }_{10} \mathrm{P}_{4}} \newline(D) (4P2)(4P2)10P4 \frac{\left({ }_{4} \mathrm{P}_{2}\right) \cdot\left({ }_{4} \mathrm{P}_{2}\right)}{{ }_{10} \mathrm{P}_{4}}

Full solution

Q. A passcode to enter a building is a sequence of 44 single digit numbers (09) (0-9) , and repeated digits aren't allowed.\newlineSuppose someone doesn't know the passcode and randomly guesses a sequence of 44 digits.\newlineWhat is the probability that they guess the correct sequence?\newlineChoose 11 answer:\newline(A) 110P4 \frac{1}{{ }_{10} \mathrm{P}_{4}} \newline(B) 110C4 \frac{1}{{ }_{10} \mathrm{C}_{4}} \newline(C) 4P410P4 \frac{{ }_{4} \mathrm{P}_{4}}{{ }_{10} \mathrm{P}_{4}} \newline(D) (4P2)(4P2)10P4 \frac{\left({ }_{4} \mathrm{P}_{2}\right) \cdot\left({ }_{4} \mathrm{P}_{2}\right)}{{ }_{10} \mathrm{P}_{4}}
  1. Understand the problem: Understand the problem.\newlineWe need to calculate the probability of guessing a correct sequence of 44 digits where no digit repeats and each digit can be from 00 to 99.
  2. Calculate total sequences: Calculate the total number of possible 44-digit sequences without repetition.\newlineSince the first digit can be any of the 1010 digits (009-9), the second digit can be any of the remaining 99 digits, the third can be any of the remaining 88 digits, and the fourth can be any of the remaining 77 digits, we use the permutation formula for this.\newlineTotal number of sequences = 10×9×8×7=504010 \times 9 \times 8 \times 7 = 5040\newlineThis is also denoted as 10P4_{10}P_{4}, which is the number of permutations of 1010 items taken 44 at a time.
  3. Calculate probability: Calculate the probability of guessing the correct sequence.\newlineSince there is only one correct sequence, the probability of guessing it correctly is 11 divided by the total number of possible sequences.\newlineProbability = 110P4=15040\frac{1}{_{10}P_{4}} = \frac{1}{5040}
  4. Match with options: Match the calculated probability with the given options.\newlineThe calculated probability matches option (A), which is 110P4\frac{1}{{}_{10}P_{4}}.

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