In a grouped data of heights, the frequencies of four consecutive classes are given below, with one frequency missing.
Class interval (cm): 40–50 50–60 60–70 70–80
Frequency: 10 x 20 12
If the mode of the distribution is 66 cm, find the value of x.
(a) 12
(b) 6
(c) 8
(d) 17