the average of 3 numbers is 7. Two of the numbers are 8 and 9. What is the 3rd?
$${\frac{\left({\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}\right)}{{\mathtt{3}}}} = {\mathtt{7}}$$
$${\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}} = {\mathtt{21}}$$
$${\mathtt{x}} = {\mathtt{21}}{\mathtt{\,-\,}}{\mathtt{8}}{\mathtt{\,-\,}}{\mathtt{9}}$$
$${\mathtt{x}} = {\mathtt{4}}$$
.$${\frac{\left({\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}}\right)}{{\mathtt{3}}}} = {\mathtt{7}}$$
$${\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{8}}{\mathtt{\,\small\textbf+\,}}{\mathtt{9}} = {\mathtt{21}}$$
$${\mathtt{x}} = {\mathtt{21}}{\mathtt{\,-\,}}{\mathtt{8}}{\mathtt{\,-\,}}{\mathtt{9}}$$
$${\mathtt{x}} = {\mathtt{4}}$$
