Inside the Loop: Only One Is In
Read the puzzle
Grid points run 0 to 8 across (left to right) and 0 to 8 down (top to bottom). One closed wall joins, in order: (1,0) (2,0) (2,2) (4,2) (4,3) (3,3) (3,4) (5,4) (5,3) (7,3) (7,4) (8,4) (8,5) (7,5) (7,7) (6,7) (6,8) (5,8) (5,7) (2,7) (2,6) (1,6) (1,5) (0,5) (0,3) (1,3), back to (1,0). Gems sit at square centres: A (0.5,3.5), B (7.5,3.5), C (1.5,6.5), D (7.5,5.5). Which gem is inside the loop?
Which gem is INSIDE the loop?
Count wall crossings on the way to the edge: odd means inside.
A
A line from gem A to the edge crosses the wall an odd number of times; from B, C or D, an even number.
How to solve it
Count crossings to the edge. Pick the shortest straight line from a gem to the border and count how many times it crosses the wall: odd means inside, even (zero included) means outside.
Gem A: go left. The wall runs down column 0 from row 3 to row 5, right where A sits, so the line crosses it exactly once. Odd, so A is inside the loop.
Gem B: go right. The wall on column 8 only covers rows 4 to 5, and B is at row 3.5, so nothing is crossed. Zero, outside.
Gem D: go right as well. D is at row 5.5, below that same short wall, so again zero crossings. Outside.
Gem C: go down. Every wall below C lies at column 2 or further right, and C is at column 1.5. Zero crossings, outside.
B and D are the usual traps: they sit tucked against the bump on the right and look enclosed, but each lies just above or below it. Takeaway: count crossings rather than judging by how surrounded a gem looks.
Learn the method: How to Solve Loop Puzzles: The Inside and Outside Method
