Tính. a. 3 / 10 + 2 / 5 b. 1 / 8 x 1 / 4 c. 2 / 5 - 1 / 4

* Hướng dẫn giải

$\mathrm{a}. \frac{3}{10}+\frac{2}{5}=\frac{3}{10}+\frac{4}{10}=\frac{7}{10}$

$\mathrm{b}. \frac{2}{5}-\frac{1}{3}=\frac{1x1}{8x4}=\frac{1}{32}$

$\mathrm{c}. \frac{1}{8}\mathrm{x}\frac{1}{4}=\frac{6}{15}-\frac{5}{15}=\frac{1}{15}$

$\mathrm{d}. \frac{1}{7}:\frac{3}{4}=\frac{1}{7}\mathrm{x}\frac{4}{3}=\frac{4}{21}$