Chiều rộng căn phòng:

8:2=4(m)

Diện tích nền phòng:

8 x 4= 32(m2)= 320000(cm2)

Diện tích 1 viên gạch :

20 x 20 = 400(cm2)

Để lát nền phòng thì cần số viên gạch là:

320 000 : 400 = 800 (viên)

Phân số trên đã là phân số tối giản rồi 

$a.\left(\genfrac{}{}{0ex}{}{5}{7}+\genfrac{}{}{0ex}{}{2}{3}\right)\times \genfrac{}{}{0ex}{}{7}{11}=\genfrac{}{}{0ex}{}{29}{21}\times \genfrac{}{}{0ex}{}{7}{11}=\genfrac{}{}{0ex}{}{29}{33}$

$C2:\genfrac{}{}{0ex}{}{5}{7}\times \genfrac{}{}{0ex}{}{7}{11}+\genfrac{}{}{0ex}{}{2}{3}\times \genfrac{}{}{0ex}{}{7}{11}=\genfrac{}{}{0ex}{}{5}{11}+\genfrac{}{}{0ex}{}{14}{33}=\genfrac{}{}{0ex}{}{29}{33}$

$b.\left(\genfrac{}{}{0ex}{}{1}{5}+\genfrac{}{}{0ex}{}{3}{9}\right)\times \genfrac{}{}{0ex}{}{2}{3}=\genfrac{}{}{0ex}{}{8}{15}\times \genfrac{}{}{0ex}{}{2}{3}=\genfrac{}{}{0ex}{}{16}{45}$

$C2:\genfrac{}{}{0ex}{}{1}{5}\times \genfrac{}{}{0ex}{}{2}{3}+\genfrac{}{}{0ex}{}{3}{9}\times \genfrac{}{}{0ex}{}{2}{3}=\genfrac{}{}{0ex}{}{2}{15}+\genfrac{}{}{0ex}{}{2}{9}=\genfrac{}{}{0ex}{}{16}{45}$
 

$a.\left(\genfrac{}{}{0ex}{}{5}{7}+\genfrac{}{}{0ex}{}{2}{3}\right)\times \genfrac{}{}{0ex}{}{7}{11}=\genfrac{}{}{0ex}{}{29}{21}\times \genfrac{}{}{0ex}{}{7}{11}=\genfrac{}{}{0ex}{}{29}{33}$

$C2:\genfrac{}{}{0ex}{}{5}{7}\times \genfrac{}{}{0ex}{}{7}{11}+\genfrac{}{}{0ex}{}{2}{3}\times \genfrac{}{}{0ex}{}{7}{11}=\genfrac{}{}{0ex}{}{5}{11}+\genfrac{}{}{0ex}{}{14}{33}=\genfrac{}{}{0ex}{}{29}{33}$

$b.\left(\genfrac{}{}{0ex}{}{1}{5}+\genfrac{}{}{0ex}{}{3}{9}\right)\times \genfrac{}{}{0ex}{}{2}{3}=\genfrac{}{}{0ex}{}{8}{15}\times \genfrac{}{}{0ex}{}{2}{3}=\genfrac{}{}{0ex}{}{16}{45}$

$C2:\genfrac{}{}{0ex}{}{1}{5}\times \genfrac{}{}{0ex}{}{2}{3}+\genfrac{}{}{0ex}{}{3}{9}\times \genfrac{}{}{0ex}{}{2}{3}=\genfrac{}{}{0ex}{}{2}{15}+\genfrac{}{}{0ex}{}{2}{9}=\genfrac{}{}{0ex}{}{16}{45}$
 

A. 320