Are we to assume that semi-dwarf lemon tree is half of a full size tree? You may have left that out of the problem.
If so, we know that a semi-dwarf is only 13.5 inches, because it is half of a full size.
Now, let the dwarf lemon tree equal x, and we have the equation.
x = (1/2)semi-dwarf
x = (1/2)(13.5)
x = 6.75
The dwarf lemon tree is 6.75, semi-dwarf is 13.5 and the full size is 27.