Question 358974
Let the number be represented by xy.
From the given information, we get the system
{{{matrix(2,1,(10x + y)/(x+y) = 7, (10y + x)/(x+y) = 4)}}}.
After simplification, the two equations yield exactly the same equation:
{{{x = 2y}}}.  Therefore there are four possible answers: 21,42,63, and 84. (Incidentally the last three numbers are just multiples of 21 =))