每次抓后不放回:恰好抓到2个红球的概率是C(10,2)*C(90,18)/C(100,20)≈0.318至少抓到2个红球的概率是1-C(90,20)/C(100,20)-C(10,1)*C(90,19)/C(100,20)≈1-0.095-0.268≈0.637-----------每次抓后放回:恰好抓到2个红...