Out of 6, the chance for a 1 or a 5 is 2. Probability is the something is the number of desired outcomes over the number of all possible outcomes. Therefore the probability is 2/6 to roll a 1 or a 5. From here, we need to find how much 2/6 of 100 is.
2/6 = x/100
200/6 = x
33.3 = x
Since its impossible to roll part of a dice, we'll round to a whole number. 33.3 rounds to 33, therefore he will roll a 1 or a 5 33 times.