$\begin{array}{l}
Cau\,\,\,1:\,\,\\
Goi\,\,\,so\,\,\,co\,\,3\,\,\,chu\,\,\,so\,\,khac\,\,nhau\,\,\,duoc\,\,tao\,\,tu\,\,tap\,\,so\,\,da\,\,cho\,\,\,\overline {abc} .\\
\Rightarrow \overline {abc} < 300 \Leftrightarrow a < 3 \Rightarrow a \in \left\{ {1;\,\,2} \right\}\\
+ )\,\,\,a = 1 \Rightarrow b,\,\,\,c\,\,\,co:\,\,\,A_4^2\,\,\,cach\,\,chon.\\
+ )\,\,a = 2 \Rightarrow b,\,\,\,c\,\,\,co:\,\,\,A_4^2\,\,\,cach\,\,chon.\\
Vay\,\,co:\,\,\,2.A_4^2 = 24\,\,\,duoc\,\,chon\,\,\,nho\,\,hon\,\,300.\\
Tu\,\,tap\,\,so\,\,da\,\,cho\,\,ta\,\,lap\,\,duoc:\,\,A_5^3 = 60\,\,\,so\,\,co\,\,3\,\,chu\,\,so\,\,khac\,\,nhau.\\
\Rightarrow co:\,\,60 - 24 = 36\,\,\,so\,\,thoa\,\,man\,\,\,bai\,\,toan.\\
Cau\,\,2:\\
Cu\,\,noi\,\,2\,\,dinh\,\,duoc\,\,1\,\,doan\,\,thang\\
\Rightarrow So\,\,doan\,\,thang\,\,duoc\,\,tao\,\,boi\,\,2\,\,diem\,\,la:\,\,\,C_n^2.\\
Trong\,\,co\,\,co\,\,n\,\,canh\,\,duoc\,\,noi\,\,boi\,\,cac\,\,dinh\,\,lien\,\,tiep\,\,cua\,\,da\,\,giac.\\
\Rightarrow co\,\,C_n^2 - n\,\,\,duong\,\,cheo.
\end{array}$
