Problem:
In how many ways can the digits , , , and be ordered so as to form a five-digit number divisible by ?
Problem:
In how many ways can the digits , , , and be ordered so as to form a five-digit number divisible by ?
Pick one
Solution:
The answer is (D). A number is divisible by if and only if the difference between the sum of the digits in odd position and the sum of the digits in even position is divisible by .
Let be the sum of the digits in odd position and the sum of the digits in even position. Since is odd, will also be odd and hence different from . Moreover, from the fact that is at most and at least , it follows that lies between and . If must be divisible by , the only possibility is therefore that it equals , but then , from which . The only pair of digits among those considered giving sum is .
In conclusion, one obtains a multiple of every time, and only when, the digits in even position are and . In total there are combinations, because there are ways to fix the order of the digits in even position and ways to fix the order of the other .