Đáp án:
$\begin{array}{l}
a)8 = {2^3};\,\,10 = 2.5;\,\,\,20 = {2^2}.5\\
\Rightarrow BCNN\left( {8;10;20} \right) = {2^3}.5 = 40\\
f)56 = {2^3}.7;\,\,\,70 = 2.5.7;\,\,\,126 = {2.3^2}.7\\
\Rightarrow BCNN\left( {56;70;126} \right) = {2^3}{.3^2}.5.7 = 2520\\
b)16 = {2^4};\,\,\,\,24 = {2^3}.3\\
\Rightarrow BCNN\left( {16;24} \right) = {2^4}.3 = 48\\
c)60 = {2^2}.3.5;\,\,\,140 = {2^2}.5.7\\
\Rightarrow BCNN\left( {60;140} \right) = {2^2}.3.5.7 = 420\\
g)28 = {2^2}.7;\,\,\,\,20 = {2^2}.5;\,\,\,30 = 2.3.5\\
\Rightarrow BCNN\left( {28;20;30} \right) = {2^2}.3.5.7 = 420
\end{array}$