Question 28
A grid 20 squares tall and 21 squares wide has every square painted green or gold. The diagram shows its top-left corner. The pattern obeys these rules:
- Every square in the leftmost column is gold.
- Only the second-leftmost square in the top row is green.
- In every L-shaped triplet consisting of two horizontally adjacent squares and the square immediately below the right-hand one, the number of gold squares is odd. The orientation is shown separately.
How many of the $20\times21=420$ squares are green?
- Every square in the leftmost column is gold.
- Only the second-leftmost square in the top row is green.
- In every L-shaped triplet consisting of two horizontally adjacent squares and the square immediately below the right-hand one, the number of gold squares is odd. The orientation is shown separately.
How many of the $20\times21=420$ squares are green?