Maths(代數2)

Let X be the tens digit and Y be the unit digit

{ X+Y=7--------1
{10(X+2)+(Y+2)-3=2(10X+Y)--------2
10X+20+Y+2-3=20X+2Y-----------2
10X+Y+19=20X+2Y------------2
Subsititue 1into 2
10(7-Y)+y+19=20(7-Y)+2Y
70-10Y+Y+19=20(7-Y)+2Y
89-11Y=20(7-Y)+2Y
89=140-20Y+2Y
89=140-18Y
18Y=140-89
Y=3

2006-11-20 17:51:59 補充：
Subsitute Y=3 into 1X+(3)=7X=4Therefore, the original number is 43.Sorry, i just forget about to subsitute it....

let (10a+b) be the number,a is the ten digit and b is the unit digit
a+b=7
b=7-a
10(a+2)+b+2+3=2(10a+b)
10a+20+b+5=20a+2b
10a+b=25
the number is 25