a. ($\frac{3}{20}$ + $\frac{1}{2}$ -x) : $\frac{32}{9}$ = $\frac{21}{128}$
($\frac{13}{20}$ -x) : $\frac{32}{9}$ = $\frac{21}{128}$
$\frac{117}{640}$ - $\frac{9x}{32}$ = $\frac{21}{128}$
$\frac{9x}{32}$ = $\frac{3}{160}$
9x = $\frac{3}{5}$
x = $\frac{1}{15}$
b. $\frac{7}{15}$ < $\frac{x}{40}$ < $\frac{8}{15}$
$\frac{56}{120}$ < $\frac{3x}{120}$ < $\frac{64}{120}$
56<3x<64
$\frac{56}{3}$ < x < $\frac{64}{3}$
Chúc bạn học tốt !!!!!!!!!
