The mean of observations, given in the adjoining table, is 20, the value of a is :

x	f	fx
10	10
20	a
30	10

1. 20

2. 30

3. 10

4. 25

By formula,

Mean = $\dfrac{Σfx}{Σf}$

Substituting values we get :

$\Rightarrow 20 = \dfrac{400 + 20a}{20 + a}$

⇒ 20(20 + a) = 400 + 20a

⇒ 400 + 20a = 400 + 20a

The above equation cannot be solved, so the question is incorrect.