A6. 2143 and 3421 are two examples of numbers that can be formed by using each of the digits 1, 2, 3, and 4 exactly once.

If you add up all the numbers that can be formed by using each of the digits 1, 2, 3, and 4 exactly once, the answer is:
(A) 5555
(B) 9999
(C) 11110
(D) 33330
(E) 66660
A6. By using the digits 1, 2, 3, and 4 exactly once, you can create 24 different numbers. If you list them one below the other, each of the four digits 1, 2, 3, and 4 appears exactly six times in the units place. This adds up to 60. The digits 1, 2, 3, and 4 also appear exactly six times in the tens place. This contributes 600 to the sum. Similarly for the hundreds and thousands places. The digits in the hundreds place thus contribute 6000, and the digits in the thousands place contribute 60000. Total: 66660. (E)