1abc = abc x 9
abc x 9 = 1abc
abc x 9 = 1000 + abc
abc x 8 = 1000 ( bớt 2 vế đi abc )
abc = 1000 : 8
abc = 125
1abc = abc x 9
1000 + abc = abc x 9
1000 = abc x 8
1000 : 8 = abc
125 = abc
vậy abc = 125
1000 + abc = abc * 9
<=> abc x 9 = 1000 + abc
<=> abc x 9 - abc = 1000
<=> 8 x abc= 1000
<=> abc = 1000/8
<=> abc = 125
1000+abc=abcx9
1000=abcx8
1000:8=abc
125=abc
k nhé