\[\begin{array}{l}
A = \left\{ {0;\,\,1;\,\,2;\,\,3;\,\,4;\,\,5;\,\,6;\,\,7} \right\}\\
a)\,\,\,Goi\,\,\,so\,\,can\,\,tim\,\,co\,\,dang:\,\,\overline {{a_1}{a_2}{a_3}{a_4}{a_5}} .\\
Ta\,\,co:\,\,{a_1} \ne 0 \Rightarrow {a_1}\,\,co\,\,\,7\,\,\,cach\,\,chon.\\
{a_2},\,\,{a_3},\,\,{a_4},\,\,{a_5}\,\,\,deu\,\,co\,\,8\,\,cach\,\,chon.\\
\Rightarrow co\,\,{7.8^4}\,\,\,so\,\,co\,\,5\,\,\,\,\,chu\,\,so.\\
b)\,\,Goi\,\,\,so\,\,can\,\,tim\,\,co\,\,dang:\,\,\overline {{a_1}{a_2}{a_3}{a_4}{a_5}} .\\
Ta\,\,co:\,\,{a_1} \ne 0 \Rightarrow {a_1}\,\,co\,\,\,7\,\,\,cach\,\,chon.\\
{a_2} \ne {a_1} \Rightarrow {a_2}\,\,\,co\,\,7\,\,cach\,\,chon.\\
{a_3}\,\,\,co\,\,6\,\,cach\,\,chon.\\
{a_4}\,\,\,co\,\,5\,\,cach\,\,chon.\\
{a_5}\,\,\,co\,\,4\,\,cach\,\,chon.\\
\Rightarrow co\,\,\,7.7.6.5.4 = 5880\,\,cach\,\,chon.\\
c)\,\,\,Goi\,\,\,so\,\,can\,\,tim\,\,co\,\,dang:\,\,\overline {{a_1}{a_2}{a_3}{a_4}{a_5}} .\\
TH1:\,\,\,{a_1} = 2 \Rightarrow {a_1}\,\,co\,1\,\,cach\,\,chon.\\
Gia\,su\,\,{a_2} = 3 \Rightarrow {a_2}\,\,co\,\,1\,\,cach\,\,\,chon.\\
\Rightarrow {a_3}\,\,co\,\,6\,\,cach\,\,chon.\\
{a_4}\,\,co\,\,5\,\,cach\,\,chon.\\
{a_5}\,\,co\,\,4\,\,cach\,\,chon.\\
Tuong\,\,tu\,\,cac\,\,so\,\,{a_3};\,\,{a_4};\,\,{a_{5\,\,}}co\,\,the\,\, = 3\\
\Rightarrow co\,\,\left( {1.1.6.5.4} \right).4 = 480\,\,so\,\,thoa\,\,man.\\
TH2:\,\,{a_1} = 3 \Rightarrow TH\,\,nay\,\,\,tuong\,\,tu\,\,TH1\\
\Rightarrow co\,\,480\,\,so\,\,thoa\,\,man.\\
TH3:\,\,{a_1} \notin \left\{ {2;\,3} \right\} \Rightarrow {a_1}\,\,\,co\,\,5\,\,cach\,\,chon.\\
Gia\,\,su\,\,{a_2},\,\,{a_3} \in \left\{ {2;\,\,3} \right\} \Rightarrow moi\,\,so\,\,\,co\,\,2\,\,cach\,\,chon.\\
Cac\,\,so\,\,con\,\,lai\,\,co\,\,5\,\,cach\,\,chon.\\
\Rightarrow co\,\,5.2.5 = 50\,\,cach\,\,chon.\\
Tuong\,\,tu\,\,ta\,\,co\,\,the\,\,lap\,\,lai\,\,voi\,\,\,\left\{ {{a_2};\,\,{a_4}} \right\} = \left\{ {2;\,\,3} \right\};\,\,\,\left\{ {{a_2};\,\,{a_5}} \right\} = \left\{ {2;\,\,3} \right\};\,\left\{ {{a_3};\,\,{a_4}} \right\} = \left\{ {2;\,\,3} \right\};\,\,\left\{ {{a_3};\,\,{a_5}} \right\} = \left\{ {2;\,\,3} \right\};\,\,\left\{ {{a_4};\,\,{a_5}} \right\} = \left\{ {2;\,\,3} \right\}\\
\Rightarrow co\,\,6.50 = 300\,\,cach\,\,chon.\\
Nhu\,\,vay\,\,co:\,\,480 + 480 + 300 = 1260\,\,so\,\,thoa\,\,man.
\end{array}\]
