Đáp án:
$\begin{array}{l}
f)\frac{{{2^7}{{.9}^2}}}{{{3^3}{{.2}^5}}} = {2^2}.\frac{{{3^4}}}{{{3^3}}} = 4.3 = 12\\
k)12,7 - 17,2 + 199,9 - 22,8 - 149,9\\
= 12,7 - \left( {17,2 + 22,8} \right) + \left( {199,9 - 149,9} \right)\\
= 12,7 - 40 + 50\\
= 12,7 + 10\\
= 22,7\\
e)\frac{x}{{10}} = \frac{y}{5};\frac{y}{2} = \frac{z}{3}\\
\Rightarrow \frac{x}{{20}} = \frac{y}{{10}};\frac{y}{{10}} = \frac{z}{{15}}\\
\Rightarrow \frac{x}{{20}} = \frac{y}{{10}} = \frac{z}{{15}}\\
\Rightarrow \frac{x}{4} = \frac{y}{2} = \frac{z}{3} = \frac{{2x}}{8} = \frac{{3y}}{6} = \frac{{4z}}{{12}}\\
= \frac{{2x - 3y + 4z}}{{8 - 6 + 12}} = \frac{{350}}{{14}} = 25\\
\Rightarrow \left\{ \begin{array}{l}
x = 25.4 = 100\\
y = 25.2 = 50\\
z = 25.3 = 75
\end{array} \right.
\end{array}$
