Question

Mean average of three numbers is 10. What is the third number if two of them are 12 and 9?​

Answer 1

Answer:

let the third number be x

$average = \frac{sum \: of \: numbers}{number \: of \: numbers} \\ 10 = \frac{12 + 9 + x}{3} \\ 10 = \frac{21 + x}{3} \\ 10 \times 3 = 21 + x \\ 30 = 21 + x \\ 30 - 21 = x \\ x = 9$

I hope it is helpful...................

Answer 2

let the third number be x

\begin{gathered}average = \frac{sum \: of \: numbers}{number \: of \: numbers} \\ 10 = \frac{12 + 9 + x}{3} \\ 10 = \frac{21 + x}{3} \\ 10 \times 3 = 21 + x \\ 30 = 21 + x \\ 30 - 21 = x \\ x = 9\end{gathered}

average=

numberofnumbers

sumofnumbers

10=

3

12+9+x

10=

3

21+x

10×3=21+x

30=21+x

30−21=x

x=9

I hope it will help you all ‍‍