For the first time there will be no collision because all the slot is empty.
Now, once the first slot is filled, after then to fill the next key in the slot there is chances of collision by 1/100.
For the next key, 2/100.
So 1/100 + 2/100+.............+9/100.
= .01+.02+.03+.......+.08+.09= .45