The weekly pocket money (in rupees) of 36 students is grouped as follows:
Class interval: 40–50, 50–60, 60–70, 70–80, 80–90
Frequency (fi): 5, 9, 12, 7, 3
Using 70 as an assumed mean, determine the mean pocket money of the students (correct to two decimal places).
a) 64.44
b) 69.33
c) 63.33
d) 68.33