A grouped data set is divided into three unequal class intervals:
Class 10–16 (width 6): rectangle height 2.5 on the frequency-density axis;
Class 16–28 (width 12): rectangle height 1.75 on the frequency-density axis;
Class 28–38 (width 10): rectangle height not shown.
If the total number of observations is 60, determine the frequency of the class 28–38.