For the following distribution, one of the frequencies is missing. If the arithmetic mean is 11, find the missing frequency.
C.I. 0-4, 4-8, 8-12, 12-16, 16-20, 20-24
Frequencies: 3, 10, ?, 14, 7, 1
a) 15
b) 11
c) 9
d) 12