The most famous geyser in the​ world, Old Faithful in Yellowstone National​ Park, has a mean time between eruptions of 85 minutes. If the interval of time between the eruptions is normally distributed with standard deviation 21.25 ​minutes

On a certain​ day, suppose there are 23 time intervals for Old Faithful. Treating these 23 eruptions as a random​ sample, there is a 0.20 likelihood that the mean length of time between eruptions will exceed what​ value?