练习册P54
25题
x=5 y=-4
练习册P55; 38题
120个学生付 $2; 80个学生支付了$1
练习册 P54; 26题
x=1/3, y=1/2
练习册P55; 题40
X12小时,Y6小时
练习册 P54; 27题
x=3/4, y=-1/9
练习册P55; 题42
$7, $1
练习册P54; 28题
x=-1, y=-1
练习册P55; 题41
39
练习册P54; 30题
x=-1, y=8
练习册P56 ; 题43
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