The average of 6.5, 10.7, 20.2

The average of 6.5, 10.7, 20.2 and $a$ is 15.

Step 1: Use the formula of average

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

Here, number of observations = 4.

So,

$ \frac{6.5 + 10.7 + 20.2 + a}{4} = 15 $

Step 2: Simplify

$ 6.5 + 10.7 + 20.2 + a = 60 $

$ 37.4 + a = 60 $

$ a = 60 - 37.4 $

$ a = 22.6 $

Correct Answer: 22.6