Explanation

The mean of 5 observations is 10.

This means the sum of the 5 observations is:

$$\text{Sum of 5 observations}=\text{Mean}\times \text{Number of observations}=10\times 5=50$$

Now, a new observation (10) is added. The new sum becomes:

$$\text{New sum}=50+10=60$$

The new mean of the 6 observations is:

New mean=New sum/Number of observations

​=660​

=10