When nine doughnuts price as much as the other ten (A reflection)


Prior to this post, I went to a doughnut seller near my dorm. I ask the seller (her) how much the doughnut cost. She said, “It is Rp.2000,- for 3 doughnuts”. Since I brought Rp.3000,- in my pocket, then I meant to buy 4 doughnuts with the 3000. Surprisingly, she gave me 5 doughnuts.

The following day, I again visited her shop and brought Rp.5000,- for doughnuts. Still, I got 8 instead of 7 (as I expected).

I started to think, how if I bought 6000,- with twice buyings? It benefits me one more doughnut compared to buying it at at a time. So I could get 5 doughnuts in the  first buying, and the other 5 in the next buying. This means, with Rp.6000,- I could have 9 or 10 doughnuts. Just in case the whole consumers of her do this tricky actions, I do believe that the seller could be disserved.

Let’s think it mathematically:
– IF the price of 3 doughnuts is Rp.2000,- THEN the unit cost of the doughnut is supposed to be 2000/3=667 or around Rp.700,- (rounded up to prevent loss).
– THUS, with Rp.3000,-, a consumer could only get 4 instead of 5 doughnuts. Unless, the seller will have around Rp.500,- loss. The problem is actually in working with non-divisible number and its rounding off.

This is not the only a case. A bakwan seller near my school ever made a rule like the following:
bakwanSome of my friends who realized this strange rule told me, “We better bought Rp.500,- for 2 bakwans twice instead of buying Rp.1000,- at once”.
What a poor Bakwan Seller..!

Well, as a mathematician (at least, the one who like to study math), I felt embarrassed. To reflect, this shows how our mathematics education fails to encourage several of its outputs.

This might be seen trivial for several groups of people, since it still happen in small scales. But to remember, this is only the observable cases. Who knows that the same case also happen in a larger unobserved area.

For mathematics educators, let’s fix this situation.
HOW? Do a revolution as we can. No matter how small the change we could make, but let’s do it.
Get our students engaged, involved, interested in math and realized how math is very meaningful and closed to them.

Let’s DO PMRI..!!


1 Comment

  1. Pingback: Start from “Common Sense”, End with “Understanding” | Math on Journey

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