30. La suma de las fracciones $\frac{5}{14}$ y $\frac{8}{21}$ vale

a) ${}^{20}\!\!\diagup\!\!{}_{28}\;$

b) ${}^{31}\!\!\diagup\!\!{}_{42}\;$

c) ${}^{40}\!\!\diagup\!\!{}_{54}\;$

La suma pedida vale:

$\frac{5}{14}+\frac{8}{21}=\frac{5}{7\times 2}+\frac{8}{7\times 3}=\frac{5\times 3}{7\times 2\times 3}+\frac{8\times 2}{7\times 3\times 2}=$

$=\frac{15}{42}+\frac{16}{42}=\frac{15+16}{42}=\frac{31}{42}$