Use the equation to find the value of x: 10x= 15(x-30).

x = 90°

Mountain height: 90× 10 = 900 meters.

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

It should be the comprehensive application of exercise 3.3 of p98.