In a histogram of students’ daily walking time (in minutes), the vertical axis shows frequency density (students per minute) because the class widths are unequal. The information read from the bars is:
0–20 minutes (width 20): height 0.8 20–30 minutes (width 10): height 1.5 30–40 minutes (width 10): height 1.2 40–45 minutes (width 5): height 1.6
What is the number of students whose walking time lies between 40 and 45 minutes?
a) 7
b) 8
c) 6
d) 12