I think that the question should read "Find a two-digit number ..."
If the digits are x and y, then the number is 10x + y. The difference between one half and one third is one sixth.
Hence (10x + y)/6 = x + y (the sum of the digits)
Multiply through by 6 to get 10x + y = 6x + 6y
That is 4x = 5y.
This means that x = 5 and y = 4; and the number is 54.