Đáp án:

$\begin{array}{l}
4a)48 = {2^4}.3;120 = {2^3}.3.5\\
 \Rightarrow \left\{ \begin{array}{l}
UCLN = {2^3}.3 = 24\\
BCNN = {2^4}.3.5 = 240
\end{array} \right.\\
b)54 = {2.3^3};90 = {2.3^2}.5\\
 \Rightarrow \left\{ \begin{array}{l}
UCLN = {2.3^2} = 18\\
BCNN = {2.3^3}.5 = 170
\end{array} \right.\\
c)168 = {2^3}.3.7;180 = {2^2}{.3^2}.5\\
 \Rightarrow \left\{ \begin{array}{l}
UCLN = {2^2}.3 = 12\\
BCNN = {2^3}.3.5.7 = 840
\end{array} \right.\\
1)90 = {2.3^2}.5;120 = {2^3}.3.5\\
 \Rightarrow UCLN = 2.3.5 = 30\\
BCNN = {2^3}{.3^2}.5 = 360\\
2)36 = {2^2}{.3^2};24 = {2^3}.3\\
 \Rightarrow UCLN\left( {36;24} \right) = {2^2}.3 = 12\\
3)220 = {2^2}.5.11\\
180 = {2^2}{.3^2}.5\\
 \Rightarrow UCLN = {2^2}.5 = 20\\
 \Rightarrow UC = \left\{ {1;2;4;5;10;20} \right\}\\
4)24 = {2^3}.3;32 = {2^5}\\
 \Rightarrow BCNN = {2^5}.3 = 96\\
 \Rightarrow x \in \left\{ {96;192;..} \right\}\\
Do:90 < x < 100\\
 \Rightarrow x = 96
\end{array}$