某校开设8门校本课程，其中4门课程为人文科学，4门为自然科学，学校要求学生    在高中三年内从中选修3门课程，假设学生选修每门课程的机会均等．

（1）求某同学至少选修1门自然科学课程的概率；

（2）已知某同学所选修的3门课程中有1门人文科学，2门自然科学，若该同学通过人文科学课程的概率都是$\frac{4}{5}$，自然科学课程的概率都是$\frac{3}{4}$，且各门课程通过与否相互独立．用ξ表示该同学所选的3门课程通过的门数，求随机变量ξ的概率分布列和数学期望．

    Today is the last Saturday before Christmas. Everyone in the USA 1 shopping for presents. 2 is falling. People are walking fast. They are trying to keep3 when they move from shop to shop. In the shops children are 4 at the toys. They are talking to a man called Father Christmas. He asks, "What do you want for Christmas?"

    In the USA it is warm and beautiful in summer. The trees and fields are green then. But now it is winter and 5is white.

    At 6, small lights make the houses and buildings 7. Christmas is a beautiful 8 of a year.

    Christmas Day always begins before 9. Children wake up very early in the morning. They open the 10from their parents. Then they wake up their parents, saying "Merry Christmas."

①X+5＞20     ②18-4a=6    ③X-19×2     ④Y÷3=75