Flip-Flop Gates
Read the puzzle
Boxes 1, 2 and 3 drop one at a time onto a top gate that sends them left or right. The left branch reaches a gate sending left to bin A or right to a middle gate. The right branch reaches a gate sending right to bin D or left to the same middle gate. The middle gate sends left to bin B or right to bin C. All four gates start pointing left, and each gate flips after a box passes it. Where does box 3 land?
Gates start as shown and flip after each box. Where does box 3 land?
BIN C
After boxes 1 and 2, box 3 goes left, right, right.
How to solve it
Run the boxes one at a time and flip every gate a box passes, right after it goes through.
Box 1: the top gate points left and the left gate points left, so box 1 drops into bin A. Both of those gates now flip to point right.
Box 2: the top gate now sends it right. The right gate still points left, since no box has used it yet, so box 2 goes to the middle gate, which also still points left: bin B. The top, right and middle gates flip, so the top gate is back to left and the middle gate now points right.
Box 3: the top gate sends it left, then the left gate, flipped by box 1, sends it right to the middle gate, which box 2 flipped, so it goes right again. Box 3 lands in bin C.
The usual wrong answer is A, which comes from forgetting the flips, or from flipping every gate after each box whether a box used it or not. Quick check: box 3's route is left, right, right.
