Đáp án + Giải thích các bước giải:

$\textbf{4A.}$

$\textbf{a}\bigg)$

$\begin {array}{l} & 6\cdot (x - 3) = 0
\\ & x - 3 = 0
\\ & x = 3\end{array}$

Vậy $x = 3$

$\textbf{b}\bigg)$

$\begin{array}{l} & 12x + 15 = 135
\\ & 12x = 120
\\ & x = 10 \end{array}$

Vậy $x = 10$

$\textbf{c}\bigg)$

$\begin{array}{l} & (4x + 25) : 15 = 7
\\ & 4x + 25 = 105
\\ & 4x = 80
\\ & x = 20 \end{array}$

Vậy $x = 20$

$\textbf{d}\bigg)$

$\begin{array}{l} & 225 : (20 - 5x) = 15
\\ & 20 - 5x = 15
\\ & 5x = 5
\\ & x = 1\end{array}$

Vậy $x = 1$

$\textbf{4B.}$

$\textbf{a}\bigg)$

$\begin{array}{l} & (x - 15) \cdot 8 = 0
\\ & x - 15 = 0
\\ & x = 15\end{array}$

Vậy $x = 15$

$\textbf{b}\bigg)$

$\begin{array}{l} & 6x - 45 = 27
\\ & 6x = 72
\\ & x = 12\end{array}$

Vậy $x = 12$

$\textbf{c}\bigg)$

$\begin{array}{l} & 187 : (5x + 2) = 11
\\ & 5x + 2 = 17
\\ & 5x = 15
\\ & x = 3\end{array}$

Vậy $x = 3$

$\textbf{d}\bigg)$

$\begin{array}{l} & 224 : (2x - 6) = 16
\\ & 2x - 6 = 14
\\ & 2x = 20
\\ & x = 10\end{array}$

Vậy $x = 10$