Let the original number be x y.(x at ten's place and y at units place)
Therefore, the number can also be 10x+y.
New number after interchanging the digits=y x.
This number can also be 10y+x.
Also given , x+y=12...........(1)
Subtracting equation (2) from equation (1) we get=
x+5=12 or, x=7.
Therefore, the original number is
57 AND THE OTHER NO IS 75