Jason remembers nine digits of a ten digit phone number. He remembers that the last digit is an even number. If he guesses that last digit, what is the probability that he dials the correct number?
He already knows 9 so that's out of the way. Now we have to figure out the last digit. What could it be? 0,1,2,3,4,5,6,7,8,9, or 0. The number of possibilities is the denominator and the number of times he tries is the numerator.