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JEE Main 2021 · Mathematics · Permutations and Combinations · medium

The number of four-digit numbers that can be formed using distinct digits from \1,2,3,4,5\ and are divisible by 4 is

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The number of four-digit numbers that can be formed using distinct digits from {1,2,3,4,5}\{1,2,3,4,5\} and are divisible by 44 is
  1. A. 2020
  2. B. 3030
  3. C. 2424
  4. D. 3636
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Answer: (C) 2424

A number is divisible by 44 exactly when its last two digits form a multiple of 44. Two-digit multiples of 44 with digits from {1,…,5}\{1,\dots,5\} are 12, 24, 32, 44, 5212,\,24,\,32,\,44,\,52. The pair 4444 repeats a digit, so it is not allowed, leaving 44 valid endings. For each ending, the first two places are filled from the remaining 33 digits in 3×2=63\times2=6 ways. Hence the count is 4×6=244\times6=24. Counting 4444 would give the trap value 3030.
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