Glass Overlay: Slide and Cancel
Read the puzzle
A 3 by 3 grid of glass cards; each card has nine dots numbered 1 to 9 like a phone keypad (1 top left, 9 bottom right), and lines join neighbouring dots. The heading reads: Slide and cancel, what's left? Row 3: card 1 has lines 2-3, 4-5, 5-7, 8-9; card 2 has 2-3, 4-5, 7-8, 8-9; card 3 is missing. Options: A: 4-7, 5-7, 7-8. B: 5-7, 7-8. C: 2-3, 4-5, 8-9. D: 2-3, 4-5, 5-7, 7-8, 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, slide card 1 onto card 2: any line on both cards cancels, and every other line stays.
Test it on row 1. Card 1 has 4-5, 5-6, 6-9 and 5-7; card 2 has 4-5, 2-5, 2-6 and 6-9. Lines 4-5 and 6-9 cancel, leaving 5-6, 5-7, 2-5 and 2-6, exactly the third card.
Row 3: card 1 has 2-3, 4-5, 5-7, 8-9 and card 2 has 2-3, 4-5, 7-8, 8-9. Lines 2-3, 4-5 and 8-9 are on both, so they cancel. What stays is 5-7 (the diagonal from the centre to the bottom-left dot) and 7-8 (the left half of the bottom edge). That is card B.
The traps are the other overlays. D keeps every line (simple stacking), C keeps only the shared lines, and A adds an extra line 4-7.
Quick check: each line in the answer must appear on exactly one of the two cards.
