Since "PROBABILITY" has 11 letters in it, then every letter has 1/11 chances of getting picked, so the chances of getting an O tile would be 1/11, same for getting a B tile. But if you were to get both of them consecutively, then the chances would be 1/11 of 1/11 because you have 1/11 of a chance to get and O and then 1/11 of a chance to get a B. So you would be looking for 1/11 of 1/11, which means multiplying the divisor, 11*11=121. Therefore, you should have 1/121 chance of getting an O and B tile.
