Tính. 4/7 + 8/21

a) $\frac{4}{7}+\frac{8}{21}=\frac{12}{21}+\frac{8}{21}=\frac{20}{21}$

c) $\frac{16}{39}\times \frac{13}{5}=\frac{16\times 13}{13\times 3\times 5}=\frac{16}{15}$

b) $\frac{9}{16}-\frac{7}{48}=\frac{27}{48}-\frac{7}{48}=\frac{20}{48}=\frac{5}{12}$

d) $\frac{25}{33}:\frac{25}{27}=\frac{25}{33}\times \frac{27}{25}=\frac{25\times 9\times 3}{3\times 11\times 25}=\frac{9}{11}$