解:(Ⅰ)由图可知,健康上网天数未超过20天的频率为 ,············· 2分 ∴ 健康上网天数超过20天的学生人数是 .·················· 4分 (Ⅱ)随机变量Y的所有可能取值为0,1,2.················ 5分 P(Y=0)=, ··························· 6分 P(Y="1)=" ,·························· 7分 P(Y="2)=" .·························· 8分 所以Y的分布列为 ··································· 11分 ∴ E(Y)=0×+1×+2×= .················ 13分 |