There are 10 sides, therefore there are 10 options each roll for what side could be shown. In probability, this means we put 10 as the denominator in the fraction. Now the condition is that a 1 is shown and on the dice, there is only one option for a side to have the number 1, as there is only one side with a 1, so we put 1 as the numerator of the fraction. Therefore the probability is 1 / 10