The sum of the digits in the unit place of all the 4 digit numbers formed with the help of 3, 4, 5 and 6 taken all at a time is:
(A) 18
(B) 108
(C) 432
(D) 144
Answer: (B)
Explanation: Required sum = 3!(3 + 4 + 5 + 6) = 108 [If we fix 3 of the unit place, other three digits can be arranged in ways similarly for 4, 5, 6.]
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