(1) (3C1)*{3*4/(7C2)}*{(3C2)/(7C2)}^2
(2) (3C2)*{3*4/(7C2)}^2*{3C2/(7C2)}+(3C1)*{(4C2)/(7C2)}*{(3C2)/(7C2)}^2
(3) 赤玉3個の確率は、{3*4/(7C2)}^3+(3P3)*{3*4/(7C2)}*{(4C2)/(7C2)}*{(3C2)/(7C2)} ‥(3)
赤玉4個の確率は、(3C1)*{3*4/(7C2)}^2*{(4C2)/(7C2)}+(3C2)*{(4C2)/(7C2)}^2*{(3C2)/(7C2)} ‥(4)
赤玉の個数の期待値=1*(1) + 2*(2) + 3*(3) + 4*(4)=