$\begin{equation*} \begin{cases} (a - 10)x + by = 2b &\\ ax - (b + 4)y = 2a - 20 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} a - 10 + b = 2b &\\ a - b - 4 = 2a - 20 & \end{cases} \end{equation*}$
a − 10 + b + a − b − 4 = 2b + 2a − 20
a + b + a − b − 2b − 2a = −20 + 10 + 4
−2b = −6
b = −6 : (−2)
b = 3
a − 10 + b = 2b
a = 2b − b + 10
a = b + 10
a = 3 + 10
a = 13
Ответ: a = 13; b = 3.

$\begin{equation*} \begin{cases} (a + 1)x - by = 2b &\\ ax + (b + 1)y = 5a & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -4(a + 1) + 6b = 2b &\\ -4a - 6(b + 1) = 5a & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -4a - 4 + 6b = 2b &\\ -4a - 6b - 6 = 5a & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -4a - 2b + 6b = 4 &\\ -4a - 6b - 5a = 6 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -4a + 4b = 4 | :2 &\\ -9a - 6b = 6 | :3 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} -2a + 2b = 2 &\\ -3a - 2b = 2 & \end{cases} \end{equation*}$
−2a + 2b − 3a − 2b = 2 + 2
−5a = 4
a = 4 : (−5)
a = −0,8
−2a + 2b = 2
2b = 2 + 2a
2b = 2 + 2 * (−0,8)
2b = 2 − 1,6
2b = 0,4
b = 0,4 : 2
b = 0,2
Ответ: a = −0,8; b = 0,2.