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The answer to the question 6.7.8 on page 99 of the first volume of the seventh grade math book.
6. Solution: Let the speed of car B be xkm/h and the speed of car A be (2x+20) km/h. 。

1/2 x+ 1/2(2x+20)= 298

x= 192

A 2× 192+20 = 404 km/h

7.( 1) Solution: When there is no wind, the aircraft speed is xkm/h.

(24+x)×2.8=(x-24)×3

x=696

Distance between two races: (696-24)×3=20 16km.

8. Solution: Suppose the blue cloth bought X meters and the black cloth bought (138-x) meters.

3x+5( 138-x)=540

x=75

Black138-75 = 63m

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