Question

The average of $9$ numbers is $8$. What should be added as ${10}^{th}$ number to make the average $9$?

• A. $10$
• B. $72$
• C. $18$
• D. $90$

Answer 1

The correct option is C $18$

Sum of $9$ numbers $=9\times 8=72$

Let ${10}^{th}$ number added is $x$

So, new sum will be $72+x$

Mean $=\frac{72+x}{10}$

Also given $\text{mean}=9$

Therefore, $9=\frac{72+x}{10}$

$\Rightarrow 72+x=90$

$\Rightarrow x=18$