题目

—What a beautiful day!—Yes，it's ________ that I'd like to take a walk. A . such nice weather B . so a nice weather C . such a nice weather D . so nice weather 答案：A20．在纸箱内装有10个大小相同的黑球、白球和红球，已知从箱中任意摸出1个球，得到黑球的概率是$\frac{2}{5}$，从箱中摸出2个球，至少得到1个白球的概率是$\frac{8}{15}$．（1）求箱中各色球的个数；（2）从箱中任意摸出3个球，记白球的个数为ξ，求随机变量ξ的分布列及数学期望．