Glass Overlay: Lines Cancel
Read the puzzle
A 3 by 3 grid of cards; each has nine dots numbered 1 to 9 like a keypad (1 top left, 9 bottom right), with lines joining neighbouring dots. Heading: lines cancel. Row 3: card 1 has 4-7, 3-5, 5-8, 3-6, 6-9, 8-9; card 2 has 1-2, 2-3, 2-5, 3-5, 6-9, 8-9; card 3 is missing. Options: A: 1-2, 2-3, 2-5, 5-8, 3-6, 6-9, 4-7. B: 1-2, 2-3, 2-5, 5-8, 3-6, 4-7. C: 1-2, 2-3, 3-5, 5-8, 3-6, 6-9, 8-9, 4-7. D: 3-5, 6-9, 8-9. Which card completes the grid?
Which card completes the grid?
Lines that appear on both cards disappear; all the other lines stay.
B
In each row, lines on both cards cancel and the rest stay. Row 3 gives card B.
How to solve it
Number the dots 1 to 9 like a keypad. In each row, place card 1 on card 2: a line on both cards cancels, and a line on just one card stays.
Test it on row 2. Card 1 has 2-5, 4-5, 5-9, 7-8, 8-9 and 3-6; card 2 has 1-2, 4-5, 5-9, 7-8, 3-6 and 6-8. Lines 4-5, 5-9, 7-8 and 3-6 cancel, leaving 1-2, 2-5, 8-9 and 6-8, which is the third card.
Row 3: card 1 has 4-7, 3-5, 5-8, 3-6, 6-9, 8-9 and card 2 has 1-2, 2-3, 2-5, 3-5, 6-9, 8-9. Lines 3-5, 6-9 and 8-9 cancel. What stays is 1-2 and 2-3 (the top edge), 2-5 and 5-8 (the full middle column), 3-6 (right side, upper half) and 4-7 (left side, lower half). That is card B.
The traps: D keeps only the cancelled lines, A wrongly keeps 6-9, and C is close to card 1 with the top edge added.
Quick check: no line of the answer may appear on both cards.
