Kids “lock in” on subtraction—with self-correcting locks and keys that make learning math facts a blast! Our innovative locks give kids the hands-on practice—and reinforcement—they need to master subtraction facts to 20. Kids just solve either one of the two equations on each key, then find the corresponding “answer” lock. If it’s a match, the key will turn and the lock will open! Set includes 20 locks and 39 keys; locks measure 3 1/4".