A cipher machine scrambles three-digit locker combinations. The code 481 yields one correct digit in the correct position. The code 185 yields one correct digit in the wrong position. The code 542 yields two correct digits, both in the wrong position. The code 667 yields no correct digits. What is the exact three-digit combination?

428

From 667, 6, 6, and 7 are eliminated entirely. From 481, since 1, 4, or 8 is correct in position: in 185, 1, 8, or 5 is correct out of position. Comparing 481 and 185, 8 is in the middle for both. If 8 were the correct digit in the correct position for 481, then for 185 it would be in the correct position too, but it says 'wrong position' for 185, contradiction. Thus 8 is not the middle digit. If 1 is correct in 481 (position 3), then in 185 it's in position 1 (wrong position), which fits. Let's test 542: two correct digits, wrong positions. 5, 4, 2 are all valid candidates since 6, 6, 7 are dead. Specifically, 4 and 2 must be in the code. Through logical deduction of positions, 4 is first, 2 is third, and 8 is second, yielding 428.