4. 解方程。（带“☆”的要检验。）
$15x = 3$ $x ÷ 3 = 2.1$ ☆$9.4 - x = 5.6$
$140 ÷ x = 3.5$ $15x + 6x = 168$ ☆$16 + 8x = 40$
$9x - 40 = 5$ $7.8x - 4.3x = 7$ $3x ÷ 5 = 4.8$
$0.4x - 3 × 0.9 = 2.9$ $6 + 3x = 12.9$ $15.7x + 2.3x = 72.72$

解：$15x = 3$
$15x÷15 = 3÷15$
$x = 0.2$
解：$x ÷ 3 = 2.1$
$x ÷ 3×3 = 2.1×3$
$x = 6.3$
解：☆$9.4 - x = 5.6$
$9.4 - x + x = 5.6 + x$
$5.6 + x = 9.4$
$5.6 + x - 5.6 = 9.4 - 5.6$
$x = 3.8$
检验：把$x=3.8$代入原方程，左边=$9.4-3.8=5.6$，右边=$5.6$，左边=右边，所以$x=3.8$是原方程的解。
解：$140 ÷ x = 3.5$
$140 ÷ x×x = 3.5×x$
$3.5x = 140$
$3.5x÷3.5 = 140÷3.5$
$x = 40$
解：$15x + 6x = 168$
$21x = 168$
$21x÷21 = 168÷21$
$x = 8$
解：☆$16 + 8x = 40$
$16 + 8x - 16 = 40 - 16$
$8x = 24$
$8x÷8 = 24÷8$
$x = 3$
检验：把$x=3$代入原方程，左边=$16+8×3=40$，右边=$40$，左边=右边，所以$x=3$是原方程的解。
解：$9x - 40 = 5$
$9x - 40 + 40 = 5 + 40$
$9x = 45$
$9x÷9 = 45÷9$
$x = 5$
解：$7.8x - 4.3x = 7$
$3.5x = 7$
$3.5x÷3.5 = 7÷3.5$
$x = 2$
解：$3x ÷ 5 = 4.8$
$3x ÷ 5×5 = 4.8×5$
$3x = 24$
$3x÷3 = 24÷3$
$x = 8$
解：$0.4x - 3 × 0.9 = 2.9$
$0.4x - 2.7 = 2.9$
$0.4x - 2.7 + 2.7 = 2.9 + 2.7$
$0.4x = 5.6$
$0.4x÷0.4 = 5.6÷0.4$
$x = 14$
解：$6 + 3x = 12.9$
$6 + 3x - 6 = 12.9 - 6$
$3x = 6.9$
$3x÷3 = 6.9÷3$
$x = 2.3$
解：$15.7x + 2.3x = 72.72$
$18x = 72.72$
$18x÷18 = 72.72÷18$
$x = 4.04$

5. 在括号里选择正确的方程的解并打“√”。
（1）$4x - 5 = 11$（$x = 1.5$ $x = 4$）
（2）$x - 3.5 = 2.8$（$x = 6.3$ $x = 0.7$）
（3）$6x - 3x = 18$（$x = 6$ $x = 2$）

（1）解：4x = 11 + 5
4x = 16
x = 4
$x = 1.5$ $x = 4$√
（2）解：x = 2.8 + 3.5
x = 6.3
$x = 6.3$√ $x = 0.7$
（3）解：3x = 18
x = 6
$x = 6$√ $x = 2$