Answer:
class midpoint (m) frequency(f) fm
11-13 12 3 36
13-15 14 6 84
15-17 16 9 144
17-19 18 13 234
19-21 20 f 20f
21-23 22 5 110
23 - 25 24 4 96
∑f= 40 +f ∑fm= 704+20f
now
mean = ∑fm ÷ ∑f
18= 704 + 20f
40+f
720 + 18f= 704 + 20f
16 = 2f
f = 8
For a moderately skewed distribution it is: If a frequency distribution has a symmetrical frequency curve, the mean, median and mode are equal