( 1)

Let the functional relationship between y and x be y = kx+b.

Substitute points (40, 10) and (60, 20).

40k+b= 10，60k+b=20

∴k= 1/2,b=- 10

∴y= 1/2x- 10

(2)

Let y=0

1/2x- 10=0

x=20

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