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Answer: No, but we know that your number ends in a 9 (one of 19, 29, 39, ...)
Why?
If the number is written $ab$, then the number is $10a+b$
$a\times b + (a+b) = 10a + b \Rightarrow ab + a = 10a$
$ab + a = (b+1)a$ so $b+1=10\Rightarrow b=9$ (or $a=0$)
But then $a$ could be anything!
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NRICH is part of the family of activities in the Millennium Mathematics Project.
NRICH is part of the family of activities in the Millennium Mathematics Project.