How Many Triangles?
The following problem was at a 2016 entrance test for the MIT PRIMES STEP program.
I drew several triangles on a piece of paper. First I showed the paper to Lev and asked him how many triangles there were. Lev said 5 and he was right. Then I showed the paper to Sasha and asked him how many triangles there were. Sasha said 3 and he was right. How many triangles are there on the paper? Explain.
The intended answer was 8: there were 5 triangles on one side of the paper and 3 on the other.
Most of the students didn’t think that the paper might be two-sided, but they came up with other inventive ideas. Below are some of their pictures, and I leave it to you to explain why they work. All the students who submitted these pictures got a full credit for this problem on the test.
I loved this problem; thanks Tanya!
I gave it to my university pre-service teachers this week as a warm-up exercise. I wrote about some of their ideas at my blog: https://amiealbrecht.com/2016/10/05/how-many-triangles/
I really enjoy your blog, and get lots of good ideas. Thanks again.4 October 2016, 11:15 pm