The mean of the following frequency distribution is 62.8. Find the value of p :

Classes	Frequency
0 - 20	5
20 - 40	8
40 - 60	p
60 - 80	12
80 - 100	7
100 - 120	8

We construct the following table :

$\text{Mean} = \dfrac{Σf$iui}{Σf_i} \\[1em] \Rightarrow 62.8 = \dfrac{2640 + 50p}{40 + p} \\[1em] \Rightarrow 62.8(40 + p) = 2640 + 50p \\[1em] \Rightarrow 2512 + 62.8p = 2640 + 50p \\[1em] \Rightarrow 62.8p - 50p = 2640 - 2512 \\[1em] \Rightarrow 12.8p = 128 \\[1em] \Rightarrow p = 10.

Hence, the value of p = 10.