Each key fits only one lock. There are ten locks and their keys in some order. How many times, in the worst case, do you have to try to match each lock with each key?
Hint
It’s not 55.
Answer
To determine the first lock you, need 9 tries. If none of them fits, then the last key fits.
To determine the second lock, you need 8 tries, and so on.
9+8+7+6+5+4+3+2+1=45.
