The frequencies of three class intervals in a distribution are given below. The histogram drawn using frequency density shows that the bar for 25–35 has the same height as the bar for 5–15.
Class 5–15: frequency 12
Class 15–25: frequency 18
Class 25–35: frequency ? (width 10)
Class 35–50: frequency 15
What is the missing frequency for the class 25–35?
a) 15
b) 12
c) 18
d) 10