先分段 y = 4(10 -x) + 6(20 -x) = 160 -10x<0x<=10
= 4(x -10) + 6(20 -x) = -2x + 80 10 = 4(x -10) + 6(x -20) = 10x - 160 20y <= 70160 -10x <= 70 x>=9
-2x + 80 <= 70 x>=5
10x - 160 <= 70x<=23
根据每部分x的取值范围一起分析
当y不超过70,x在什么范围内取值9<=x<=23