Knowing how forgetful she can be, Mary wrote down five clues so that she may work out the combination of her 3-digit lock.

- (7,9,3) one number is correct and correctly placed;

- (7,2,5) one number is correct but incorrectly placed;

- (3,1,7) two numbers are correct but incorrectly placed;

- (8,4,9) nothing is correct;

- (8,9,1) one number is correct but incorrectly placed.