I was hanging around at home last week, a little lost for something to do, when I got a phone call from Ernie. He was in a bit of a panic because a guest was going to drop in later in the day. He had inadvertently dismantled his oven for a certain experiment he was working on so had no way to bake a cake for the guest. "Is there any way...", he pleaded, "...that you would be able to bake something for me in your oven?". I was in a generous mood so told him I would be delighted to help and asked what time he needed the baking brought over. We settled on a time that afternoon and after checking that the guest didn't have any allergies I hung up and proceeded to the kitchen where I began to mix the ingredients.

I was just about to put the dough into a nice round tin when the phone rang again. It was Ernie. "One little thing I neglected to mention." he said. "The guest is Great Aunt Titania. Now as you know, Auntie Titania loves symmetry above all else, so you might want to make the cake in the shape of a perfect regular polygon." I winced a little - my last meeting with Titania had been a bit stressful - so I poured myself a wee brandy to calm my nerves. After a moments thought I could see a simple solution. I put the round cake tin back on the shelf, selected a more appropriately shaped one, poured in the dough, and popped the cake into the oven.

Some time later I was just turning the cake out onto a rack to cool when I got another call from Ernie. "Small change in plans" he said. " Unknown to me, Auntie Titania invited her sister Great Aunt Lusitania to drop in too. Now as you know Auntie Lusitania loves equality above all else, so to keep her happy you might want to have two cakes, both the same shape and size of course". I felt a sharp pain in my left temple, it didn't pay to leave Lusitania unsatisfied, but I still needed to keep Titania happy. I poured another brandy to dull the pain. "But I don't have enough ingredients (or time) to make a second cake" I lamented. "Oh that wont be a problem", Ernie responded, "I am sure the aunts won't mind if you cut the original cake up and reassemble the pieces into different shapes". After a little thought I could see a fairly simple solution so I made a series of straight, vertical cuts in the original cake and reassembled the pieces into two smaller identical cakes. I was quite proud that there was no waste left over when I had finished, so I poured myself a celebratory brandy.

I was just about to put the two (identical) cakes into a box to take them over to Ernie, when the phone rang again. I could tell from the ring-tone that it was Ernie, so I had a small brandy to calm my nerves before answering. "Small change in plans" he said. "Unknown to me, Auntie Lusitania invite her sister Great Aunt Bismarkia to drop in. So I am afraid we will need three cakes - and as you know, Auntie Bismarkia loves efficiency above all else. You might want to use the smallest total length of cuts possible when re-dividing the cakes ". I swigged a large drink from the bottle - my head was pounding. "But I wasn't thinking about efficiency when I made the first cuts", I lamented, "How can I possibly satisfy Bismarkia?". "Oh I am sure she will compromise just a little.", Ernie responded, "Instead of the smallest total length of cuts, why not just ensure you end up with the smallest total number of pieces instead". It took me quite a while to find a solution, then I made another series of straight, vertical cuts in the existing cake pieces and reassembled them into three still smaller identical cakes. I was quite proud that there was no waste left over when I had finished, so I poured the last of the brandy into a glass to celebrate.

I decided that driving wouldn't be the most responsible way to get the cakes to Ernie, so walked (slightly unsteadily) around to drop them off at the door. "Perfect", Ernie said as soon as he clapped his eyes on the cakes, "Regular polygons - that will make Auntie Titania happy, all three cakes are the same size and shape - that will satisfy Auntie Lusitania, and I see you have used the smallest possible total number of pieces to achieve the goal - that should get Auntie Bismarkia's approval. You made a lucky initial choice of cake-tin as no other starting polygon would use less pieces." I was offered a place at the table, but felt that the aunts might not quite approve of my somewhat slurred speech, so I headed back home.

enter image description here

Unfortunately, possibly as a result of the slight over-consumption of brandy, my mind is now a complete blank regarding the solution I found. And to be honest, the kitchen was such a mess when I woke up this morning that I couldn't even tell which polygonal cake tin I had used to bake the original cake! Can you help? The best regular polygon to start with, the minimum number of pieces required and a simple diagram should help jog my memory.

  • $\begingroup$ you want the step down to 2 and then 3 identical cakes? or just from 1 to 3? it says that when he went from 1 to 2 cakes it wasn't so efficient. $\endgroup$ – Z. Dailey Mar 8 '16 at 3:39
  • $\begingroup$ Sorry, I guess I wasn't completely clear - I think my head may still be a bit woolly from the hangover. I first cut the original cake into pieces to make two cakes - I hadn't planned that efficiently but who knows, maybe I was efficient. Then I cut those pieces up into smaller pieces sufficient to make three cakes. Ernie claims that at the end I had the smallest number of pieces that would allow me to go from 1 cake, to 2 cakes, to 3 cakes. (and that I couldn't have done better starting from any other regular polygon). $\endgroup$ – Penguino Mar 8 '16 at 20:09
  • $\begingroup$ Jeeves might be able to figure out the cuts you made! ;) $\endgroup$ – Richard Roe Mar 8 '16 at 20:28
  • $\begingroup$ Does each cut have to go all the way through the cake? i.e. if I have a polygon with an odd number of vertices, could I make a cut from each vertex to the center of the polygon? $\endgroup$ – VictorHenry Mar 8 '16 at 22:01
  • $\begingroup$ Yes a cut can go partially into the cake (so long as it is vertical and goes the full vertical depth). $\endgroup$ – Penguino Mar 10 '16 at 2:40

I'll start with a star-shaped cake which is a regular polygon. After 3 cuts the 6 parts can be arranged into 2 hexagons.

enter image description here

In the second step again 3 cuts are performed and the parts are rearranged to 3 triangles. If aligning of the parts before cutting is allowed the 3 cuts can be performed as a single cut as well.

enter image description here

  • $\begingroup$ Nicely illustrated. And I believe 9 pieces is the minimum. $\endgroup$ – Penguino Mar 9 '16 at 21:19
  • $\begingroup$ @f'' I'm not an expert on this topic. Wikipedia lists it as a regular star polygon, therefore I assumed it to be correct. $\endgroup$ – Sleafar Mar 30 '16 at 6:04

The answer is

Two cuts total

like so

the shape is irrelevant, pick any regular polygon you want. For the first, slice horizontally in half. For the second, set the two pieces next to each other and slice horizontally again, this time, a third of the way up. Take the bottom third from each and stack together. Voila, three cakes, all the same shape and size and are regular polygons.

  • 1
    $\begingroup$ This doesn't result in regular polygons. $\endgroup$ – Z. Dailey Mar 8 '16 at 5:45
  • $\begingroup$ @Z.Dailey by "horizontally", the answer means that you are cutting the thickness of the cake in half. Both halves have the same shape as before, but they are thinner. $\endgroup$ – f'' Mar 8 '16 at 7:27
  • $\begingroup$ ;) gotcha. Tricky. $\endgroup$ – Z. Dailey Mar 8 '16 at 7:32
  • $\begingroup$ That is a nice gotcha - I should have clarified that my cuts (as in the "Ernie and the cutting of the cakes" puzzle) are always vertical. This means each aunt gate an equal share of the lovely caramelized top of my cake. $\endgroup$ – Penguino Mar 8 '16 at 20:14
  • 1
    $\begingroup$ Edited so no longer shoulda. Now dida. $\endgroup$ – Penguino Mar 10 '16 at 2:41

the cake is a lie hexagon, 3 cuts, squares, ..., profit?

but if I'm wrong:

I'll be like his aunts and go down with my ship.

  • $\begingroup$ I think the triangles are slightly too wide, their short leg is $1/\sqrt{3}$ times the long leg, but they'd need to be $1/2=1/\sqrt{4}$. $\endgroup$ – 2012rcampion Mar 8 '16 at 6:42
  • $\begingroup$ Not quite - either the squares aren't squares, or the hexagon isn't regular, And remember - I cut the original cake into two smaller (regular polygonal) cakes first, so you have to take those cuts into account. $\endgroup$ – Penguino Mar 8 '16 at 20:16

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