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Explanation:
There are 7 numbers in the set {1,2,...,6,7}. Then we add on 0 to get to the 8th number. So there are 8 numbers overall listed in bold above.
Since the sample space is {0,1,2,...,8,9}, we have 10 digits listed here. So the probability of success is 8/10 = 0.80 = 80%
Side note: The values 8 and 9 would represent a failure. We have 2 failures out of 10 so we get 2/10 = 0.20 = 20%