Text version of this lesson
Totals along both axes pin a board down faster than any single clue.
Key takeaway: Always start from the most constrained row or column: the one with the fewest ways left to be filled.
Question 1
Which lights are ON?
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Why: One per row and one per column means the lights form a permutation: each choice removes a column for everyone else.
Hint: Row A has only one column left open.
Question 2
Which lights are ON?
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Why: The chain runs from the most constrained cell outward: find the row with one option left and repeat.
Hint: Column 1 is barred for rows B and C, and row A is already placed. Only one row can take it.
Question 3
Which lights are ON?
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Why: Row totals and column totals must agree — both add up to five, and that cross-check is what makes the grid solvable.
Hint: Row B needs two lights but cannot use column 1, so it takes both remaining columns.
Question 4
Which lights are ON?
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Why: Once column 1 forces a light into both remaining rows, each of them has exactly one slot left to fill.
Hint: Row B is done after its single light, so columns 1, 2 and 4 must be filled by rows A and C.
Question 5
Which lights are ON?
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Why: A column total equal to the number of rows fills the whole column at once — the strongest clue on the board.
Hint: Column 1 has as many lights as there are rows, so every row is lit there.
Question 6
Which lights are ON?
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Why: Parity turned three separate columns into three separate instructions, and the exclusion picked which row went where.
Hint: Row A holds three of the four columns, and column 4 is not one of them.