Đáp án:
$\begin{array}{l}
b2:\\
Gọi\,số\,cây\,8A,8B,8C\,là\,a,b,c \Rightarrow a + b + c = 180\\
a,b,c\,tỷ\,lệ\,với\,4:6:8\\
\Rightarrow \frac{a}{4} = \frac{b}{6} = \frac{c}{8} = \frac{{a + b + c}}{{4 + 6 + 8}} = \frac{{180}}{{18}} = 10\\
\Rightarrow \left\{ \begin{array}{l}
a = 4.10 = 40\\
b = 6.10 = 60\\
c = 8.10 = 80
\end{array} \right.\\
Vậy\,số\,cây\,3\,lớp\,lần\,lượt\,là:40;60;80\,cây\\
b3:\\
a)x - \frac{1}{4} = {2^5}:{2^3}\\
\Rightarrow x - \frac{1}{4} = {2^2} = 4\\
\Rightarrow x = 4 + \frac{1}{4}\\
\Rightarrow x = \frac{{17}}{4}\\
b)\frac{2}{3}x - \frac{4}{5} = \frac{1}{5}\\
\Rightarrow \frac{2}{3}x = \frac{1}{5} + \frac{4}{5}\\
\Rightarrow \frac{2}{3}x = \frac{5}{5} = 1\\
\Rightarrow x = 1:\frac{2}{3}\\
\Rightarrow x = \frac{3}{2}
\end{array}$
