This is a Consecutive Sudoku starting grid, and we can fill in the hilighted cells. The easiest cells to
start with in a Consecutive Sudoku are those that are 'consecutive' with a 1 or a 9 - there is only
one possible solution in this situation, either a 2 or a 8 respectively. We can use this to fill in
the top and bottom highlighted cells.
The highlighted cell to fill in on the center-right is a little trickier to fill in. We can use the
normal rules of Sudoku to eliminate 1, 2, 3, 4, 6, 7, and 8. This leaves us with just 5 and 9. We
also know that this number be must be consecutive with 6, i.e. 5 or 7. This allows us to fill this
cell in with a 5.