Text version of this lesson
One press moves several lights at once. Plan the presses, not the lights.
Key takeaway: Pressing a switch twice cancels itself, so only the set of switches you press matters, never the order.
Question 1
Pressing a light flips it and the light to its right.
Turn every light ON.
Show answer and explanation
Why: Two presses do it. Fixing the leftmost light first removes all the guesswork.
Hint: Only light 1 can be changed by a single switch. Start there.
Question 2
Pressing a light flips it together with the light on each side.
Turn every light ON.
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Why: Order never matters here: only the set of switches you pressed decides the result.
Hint: Pressing the same switch twice undoes it, so each switch is used once or not at all.
Question 3
Pressing a light flips everything in its row and everything in its column.
Leave only the light at row A, column 1 ON.
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Why: Three presses, and the bulb you never pressed is the one that stays lit.
Hint: Every press flips exactly three of the four bulbs.
Question 4
Pressing a light flips the other two, but never itself.
Leave the red and yellow lights ON, and purple OFF.
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Why: One press. The switch that leaves itself alone is exactly the one you want.
Hint: You need the two lights you are not pressing to come on.
Question 5
Pressing a light flips it and its neighbours above, below, left and right.
Turn every light ON.
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Why: Pressing the four corners lights the whole board. Symmetry in the goal points at symmetry in the solution.
Hint: The target is symmetric, so look for a symmetric set of presses.
Question 6
Pressing a light flips it and its right-hand neighbour. Light 4 is neighboured by light 1.
Leave lights 1 and 3 ON, and the rest OFF.
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Why: Two neighbouring presses shift a lit pair around the ring.
Hint: In a ring there is no end to start from, so try a pair of neighbouring switches.