Gọi số cần tìm là ab.
Theo đề bài ta có:
ab=21.(a-b)
10a+b=21a-21b
11a=22b
a=2b
Với b=1 thì a=2.
.....
Với b=4 thì a=8.
Vậy có 4 số.
Chúc em học tốt6^
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