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The answer to the first volume of seventh grade mathematics
Equation method: Let this train be x meters long.

(X- 1× 15)÷ 15 =(X+ 1× 17)÷ 17

17(X- 15)= 15(X+ 17)

17X-225= 15X+225

17X=5 10+2X

2X=5 10

X=255

12. Equation method: Let these four numbers be A, B, C and D respectively.

a+2=b-2=2c=0.5d,

a+4=b=c=d×0.5×0.5=0.25d

a+(a+4)+2(a+2)+(a+2)/2=99

4a+(a+2)/2=9 1

2[4a+(a+2)/2]=9 1×2

8a+a+2= 182

9a= 180

a=20

Solve the values of b, c, d c and d by yourself.

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