(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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