A histogram has classes:
0-10 density 2,
10-25 density 4,
25-45 density d.
The midpoints are 5, 17.5 and 35 respectively.
The mean of the whole distribution is 20.
Find the frequency of the final class.
Figure (to be drawn): Histogram with classes 0-10 (density 2), 10-25 (density 4), 25-45 (density d). Not to scale.
(Total for Question 11 is 4 marks)