A group of 15 rats running a straight-alley maze required the following number of trials to perform to a predetermined criterion. The frequency distribution follows.

Trials to reach criterion 18 19 20 21 22 23 24 Number of rats (frequency) 1 0 4 3 3 3 1 Calculate the mean and median number of trials to criterion for this group. (You can either write out the 15 numbers or you can think about how you could incorporate the frequencies directly into the formula for the mean.)