A box leaf plot has 5 points and they are:
the minimum, the first quartile (Q1), the median (Q2), the third quartile (Q3), and the maximum. The box encompasses Q1, Q2, and Q3. The maximum is after Q3 and the minimum is before Q1.
In this case,
Q1, Q2, and Q3 are in the range 20 - 30, thus the maximum comes after 30, which is 45
the maximum value is 45
