When calculating a probability, you are comparing the number of favorable outcomes to the number of possible outcomes. To calculate the probability that a randomly chosen number is prime we will use the Probability Formula.
P=Favorable Outcomes/Possible OutcomesWe have 21 integers, from 10 to 30, inclusive. Therefore, there are 21 possible outcomes.
10, 11, 12, 13, 14,
15, 16, 17, 18, 19,
20, 21, 22, 23, 24,
25, 26, 27, 28, 29,
30
Out of these, 6 are prime which is the number of favorable outcomes. Now we have enough information to calculate P(number is prime).
