Question 26725
we could probably do this by looking:


Easy possibles are 
18 and 81
27 and 72
36 and 63
45 and 54


Looking, 27+45 is 72 so 27 is your answer.



OK, mathematically...


Let tens be x and units be y --> x+y = 9

Also, (10y+x) - (10x+y) = 45
10y+x-10x-y = 45
9y-9x = 45


we have 
y+x = 9
9y-9x = 45


multiply first by 9 and we get
9y+9x = 81
9y-9x = 45


Now subtract, leaving 18x = 36. So x = 2. Therefore, from x+y=9, y must be 7.


jon.