Question 13
A number is called “lucky” if it is divisible by 2 and all of the digits are different. When swapping the first and the last digit, we get a new number which is still divisible by 2. How many 4-digit “lucky” numbers are there?
Editorial grading note: to make the intended interpretation explicit, the swapped result is also required to be a four-digit number; a leading zero is not allowed.
Editorial grading note: to make the intended interpretation explicit, the swapped result is also required to be a four-digit number; a leading zero is not allowed.