$\begin{equation*} \begin{cases} y = 5x &\\ 4x + y = 180 & \end{cases} \end{equation*}$
4x + 5x = 180
9x = 180
x = 180 : 9
x = 20
y = 5x = 5 * 20 = 100
Ответ: (20;100)

$\begin{equation*} \begin{cases} x - 2y = 5 &\\ 2x + y = 9 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x - 2y = 5 &\\ y = 9 - 2x & \end{cases} \end{equation*}$
x − 2 * (9 − 2x) = 5
x − 18 + 4x = 5
5x = 5 + 18
x = 23 : 5
x = 4,6
y = 9 − 2x = 9 − 2 * 4,6 = 9 − 9,2 = −0,2
Ответ: (4,6;−0,2)

$\begin{equation*} \begin{cases} y = -1,4x &\\ x - y = 18 & \end{cases} \end{equation*}$
x − (−1,4x) = 18
x + 1,4x = 18
2,4x = 18
x = 18 : 2,4
x = 7,5
y = −1,4x = −1,4 * 7,5 = −10,5
Ответ: (7,5;−10,5)

$\begin{equation*} \begin{cases} x - 10y = 1 &\\ 2x + 3y = 48 & \end{cases} \end{equation*}$
$\begin{equation*} \begin{cases} x = 10y + 1 &\\ 2x + 3y = 48 & \end{cases} \end{equation*}$
2 * (10y + 1) + 3y = 48
20y + 2 + 3y = 48
23y = 48 − 2
23y = 46
y = 46 : 23
y = 2
x = 10y + 1 = 10 * 2 + 1 = 20 + 1 = 21
Ответ: (21;2)