Small marsupial with large back legs and tail for hopping.

Pademelons are small marsupials

Image: ©

Fred is a biologist doing an experiment with pademelons. They have a square paddock and four feeding bowls. They also have two laser detectors that each shoot a straight beam all the way across the paddock.

By setting up the lasers in a cross, Fred can divide the paddock into four areas, one for each feeding bowl. That way they can tell if the pademelons move between bowls!

Square divided into 4 quadrants with two crossing lasers and a green dot in each quadrant.

Set up with lasers in a cross

Recently, Fred got three more feeding bowls. How many laser detectors will they have to get so they can keep all seven bowls separated by laser beams?


Scroll down for the answer!


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Brainteaser answer

Fred can make seven sections with three laser detectors, so they’ll only need to get one more.

They can space three laser detectors around the paddock, aiming them so the beams meet in the middle, to get six sections (like six slices of pizza).

Square divided into 6 areas with three crossing lasers and a green dot in each area.

Three lasers can form six sections

Moving one of the lasers slightly so it misses the centre, will make a very small seventh section in the middle. By adjusting each laser in turn, they can make that region big enough to be useful.

Square divided into 7 areas with three crossing lasers and a green dot in each area.

Three lasers can also form seven sections.

(Or for a clever answer, Fred could use mirrors to bounce the beams around, so they might not need more detectors!)

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