:4. Solution.  The total number of families involved is 15+75+60
= 150.  The percentage of families in the first class interval is <p>

(15/150)x100 = 10% <p>

The width of the class interval is 20 - 0 = 20.  10%/20 = 0.5%,
so the height of the rectangle over the first class interval
should be 0.5. <p>