Question 17
In $a = 5 \times 5$ grid, touching a cell toggles it and its edge-neighbors. Black means off, white on. Initially black cells are $\left(1 , 1\right) , \left(1 , 3\right) , \left(1 , 5\right) , \left(5 , 1\right) , \left(5 , 5\right)$, as shown. Can all lights be switched off?
