What will be the average of all the even numbers between 21 and 57 ?

Correct option is B

Concept Used: 

An even number is a whole number divisible by 2, which means there is no remainder when divided by 2.

$$S_n = \frac{n}{2} (a + a_n)$$​​​

​$$\text{Average} = \frac{\text{Sum of observations}}{\text{Number of observations}}$$​​

Solution:

=> a = 22, an = 56 [first and last even number]

Sum of even numbers between 21 and 57 = $$\frac {n}{2}[{22+56}]= 39n$$​​

Number of observations = n

Average = $$\frac{39n}{n}=39$$​