The probability is 0.0008.
I used a binomial probability on this one, since there are two outcomes (either pass or fail); the events are independent (the probability of one person passing does not influence the probability of another passing); and there are a fixed number of trials (in this case, 20).
For binomial probabilities, we use the formula:

With our information, we have:
Convert 3 hours and 30 minutes all into minutes so that would be 210 minutes. You then have 169 miles / 210 minutes. Get the denominator to 1 minutes by dividing by 210 to the top and bottom and you'll get .080476 miles / 1 minute which is the answer.