$\text{Vì số trung bình cộng là 6, 8}$
$\text{ → }$$\frac{5 . n + 6 . 5 + 9 . 2 + 10 . 1}{n + 5 + 2 + 1}$ $\text{= 6, 8}$
$\text{ → }$$\frac{5 . n + 30 + 18 + 10}{n + 5 + 2 + 1}$ $\text{= 6, 8}$
$\text{ → }$$\frac{5 . n + 58}{n + 8}$ $\text{= 6, 8}$
$\text{ → }$$\text{5 . n + 58 = (n + 8) . 6, 8}$
$\text{ → }$$\text{5 . n + 58 = 6, 8 . n + 6, 8 . 8}$
$\text{ → }$$\text{5 . n + 58 = 6, 8.n + 54, 4}$
$\text{ → }$$\text{6, 8 . n - 5 . n = 58 - 54, 4}$
$\text{ → }$$\text{1, 8 . n = 3,6}$
$\text{ → }$$\text{n = 3, 6 : 1, 8}$
$\text{ → n = 2}$