A interesting combinatorics problem for beginners.

How many four-digit number numbers can be formed which are divisible by 4 are can be formed from the digits 0,1,2,3,4(without repetition)? (Hint:- Work out the cases...)


The answer is 30.

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1 solution

Chris Lewis
Oct 9, 2020

Let A B C D \overline{ABCD} be such a number. Since it's a multiple of four, and is made up of distinct digits less than 5 5 , the digits C D \overline{CD} are one of { 04 , 12 , 20 , 24 , 32 , 40 } \{04,12,20,24,32,40\} .

If these two digits are both non-zero, then there are 2 2 choices for A A (it can't be zero), and then 2 2 choices for B B , for a total of 4 4 choices.

If one of the digits C D \overline{CD} is zero, then there are 3 3 choices for A A , and 2 2 for B B , for a total of 6 6 choices.

In the list of possible C D \overline{CD} pairs, there are three of each type; so the answer is 3 × 4 + 3 × 6 = 30 3\times 4+3\times 6=\boxed{30} .

Interesting...

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