Bạn tham khảo :
$30x +3(12 - x) = 171$
$30x + 36 - 3x = 171$
$30x - 3x +36 = 171$
$27x +36 = 171$
$27x = 135$
$x = \dfrac{135}{27} = 5$
$5(x+7) + 3(x-5) = 36$
$5x+35 + 3x - 15 = 36$
$(5x+3x) + (35 - 15) = 36$
$8x + 20 = 36$
$8x = 36 -20$
$8x = 16$
$x = \dfrac{16}{8} = 2$
