1、两只都不是蓝球的概率 = 4/7*7/9 = 4/9
故至少有一只蓝球的概率 = 1-4/9 = 5/9
2、P = 3/7*4/9 + 2/7*2*9 = 16/63
3、P = (16/63) / (5/9) = 16/35