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This is an example of a completed LightUp puzzle. Your aim is to place light bulbs in to the grid so that,

- All white cells in the grid are lit.
- One light bulb is not lighting another light bulb.
- Some of the black cells in the grid will have number clues. This clue tells you how many light bulbs are surrounding that cell.

This is a starting grid for Light Up puzzle. None of the cells are lit, and the challenge is to insert light
bulbs so that all cells are lit.

The best place to start with a new empty grid, is to look for easy clues. We can look at the '0' in the top-right
first - we can mark the two cells around this clue with a 'X'. We can also look at the '4' in the bottom-left.
There are only 4 cells around this clue, so all of them must be lightbulbs.

We are now in this position. We know that one light bulb can't light another, so we can look at the highlighted '2'
clue in the bottom-left. We have 4 cells in total around this clue, but two are already lit, so they can't
contain a light bulb. That means the two un-lit cells must be light bulbs.

We have now done all the easy straightforward clues. Let's now look at the cell marked 'A'. If we were
to put a light bulb in this cell, it would light up the two highlighted cells to the left and below. This
would then mean that the '2' clue here would be impossible to satisfy. This means that cell 'A' can't be
a lightbulb, and we can insert a 'X' in this cell.

We can also apply the same logic with cell B. If we insert a light bulb in cell B, that would light up
the two highlighted cells. That would only leave one cell available for the '2' clue, i.e. not enough.
This means that cell 'B' can't be a light bulb, and we can insert a cross.

There are many places in the puzzle where we can apply the same logic. Have a look a cells C, D, E, F, G, and H,
and try to work out why we can't insert a light bulb in to these cells.

We can now look at the highlighted '2' clue. We now only have two available cells left for this clue, i.e.
the cells above and below, so these two must be light bulbs.

Again, this leaves the highlighted '2' clue with just two available cells for light bulbs, i.e. the cells
above and to the right. We should insert light bulbs in for these two cells.

We now have a total of 4 light bulbs we can insert quite easily. The highlighted '3' clue has only 3 available
cells, so they must all contain light bulbs. Similarly, the highlighted '1' clue has only 1 available
cells, so that cell must contain a light bulb.

Looking at the highlighted '1' clue, we only have one available cell, so the cell above the '1' clue
must contain a light bulb.

We can also look at the cell marked with an 'A'. The '2' clue directly below cell A already has it's
two light bulbs, so cell 'A' must be empty - we can mark it with a 'X'.

We can also look at the highlighted '2' clue on the left side of the puzzle. This clue already has one
light bulb, but with only the cell to it's right available, that cell must also contain a light bulb.

The '2' clue on the left side of the puzzle only has two available empty cells, so we can insert light bulbs
in those.

I have also highlighted a cell on the right side of this puzzle. Towards the end of a puzzle like this,
it's often useful to look at where you can insert a light bulb to light a particular cell, like the
highlighted cell here. The only place we can insert a light bulb to light up the highlighted cell is in
the cell marked 'A'. There is nowhere else! This means cell 'A' must contain a light bulb.

Looking at cell 'A', there is no other cell here that can light up this cell apart from itself, so we
can insert a light bulb in to cell 'A'.

We can also look at cells 'B' and 'C'. We know that one of these cells must contain a light bulb, and
the other must be empty (because of the '1' clue between them).
If we place a light bulb in cell 'B', then that would mean cell 'C' would be empty. This would leave
this puzzle impossible to finish as there is no other cell that can light up cell 'C'. This means
that cell 'C' must contain a light bulb, and cell 'B' must be empty.

All that is left now is to insert a light bulb in to the highlighted cell, and our puzzle is complete!

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