Need help with the following problem,

a) One of the numbers 2, 4, 6, 7, 8, 11, 12 and 13 is chosen at random as the numerator of a fraction, and then one of the remaining numbers is chosen at random as the denominator of the fraction. What is the probability of the fraction being in lowest terms?

I know I need to get the number of elements in the event (fraction in lowest terms) over the number of elements in the sample space to find the probability, but I'm not sure which counting techniques I need to use to find the cardinality of each these sets.