không gian mẫu: $n(\Omega)= C^{4}_{10}$.a) $n(A)=C^{2}_{5}.C^{1}_{3}.C^{1}_{2} \Rightarrow $ xác suất:$P=\frac{n(A)}{n(\Omega )}=...$
b) TH1:1dễ,1 khó,1tb: $\Rightarrow C^{1}_{5}.C^{1}_{3}.C^{2}_{2}=n1$
TH2:1 dễ,2 khó, 1tb:$\Rightarrow C^{1}_{5}.C^{2}_{3}.C^{1}_{2}=n2$
TH3:2 dễ,1 khó ,1 tb: $\Rightarrow C^{2}_{5}.C^{1}_{3}.C^{1}_{2}=n3$
$\Rightarrow $ xác suất $P=\frac{n1+n2+n3}{n(\Omega )}$