(20 10? In recent years, coal mine safety accidents have occurred frequently in China, among which gas is the most harmful, and its main component is carbon monoxide.

(1) Because the concentration increases linearly before explosion,

Therefore, the functional relationship between y and x can be set as y=k 1x+b(k 1≠0).

From the image, we know that y=k 1x+b passes through (0,4) and (7,46).

Then b = 47k 1+b = 46,

The solution is k 1 = 6b = 4,

Then y=6x+4, and the range of independent variable x is 0 ≤ x ≤ 7.

(If you don't take x=0, you won't be penalized, and x=7 can be put in the second function. )

After the explosion, the concentration decreased inversely.

∴ Let the functional relationship between y and x be y = K2x (K2 ≠ 0).

As can be seen from the image, Y = K2x passes through the point (7, 46),

∴k27=46，

∴k2=322，

∴ y = 322 x, and the value range of the independent variable x is x > 7.

(2) When y=34, it is given by y=6x+4, 6x+4=34, and x = 5.

∴ The longest evacuation time is 7-5=2 (hours).

∴ The minimum evacuation speed is 3 ÷ 2 = 1.5 (km/h).

(3) When y=4, it is obtained from y=322x, and x=80.5.

80.5-7=73.5 (hours).

At least 73.5 hours after the explosion, the miners can go down the well.