设y=kx+b
把(40,2)(50,4)
{40k+b=2
50k+b=4
解之得{k=1/5
b=-6
∴y=1/5x-6
(2)令y=0
0=1/5x-6
x=30
∴m=30
(3)令y≤30
1/5x-6≤30
x≤180