A combination lock with three dials, each numbered 1 through 8, is defective in that you only need to get two of the numbers right to open the lock. (For example, suppose the true combination is 4-2-7. Then 4-2-7 would open the lock, but so would 425422827 or 4-6-7. But not 2-4-7.)

Hint: Call the numbers 1 through 4 small," 5 through 8 "big." Observe that the true combination must have either two (or more) small numbers or two (or more) big numbers.