A histogram represents the daily rainfall (in mm) recorded in a town during a month of 30 days. All class widths are 10 mm.
0–10 mm: 8 units 10–20 mm: 12 units 20–30 mm: 5 units 30–40 mm: 5 units
How many days had rainfall between 10 mm and 30 mm?
a) 22
b) 17
c) 18
d) 12