Note that the probability of rolling a sum of 12 on two dice is 1/36. What is the least number of times a pair of dice needs to be rolled in order for the probability of obtaining a sum of 12 at least once to be at least 0.9?
Every time you roll the dice, you have a 1/36 change of getting 12. If you roll the dice a certain number of times, the probability is multiplied by the number of times you roll the dice.
1/36 = .028
and we need to find the smallest number of times we can roll so that the fraction is at or above .9 turns out 33/36 is .916 so if you roll the dice 33 times, your probability of rolling a 12 is at least .9