The Conversation (0)
News
Last week people across the globe were left stumped by "Cheryl's birthday" - a maths puzzle used to test 15 to 16-year-old students in Singapore.
And now, for those of you who found that easy (i.e. almost precisely no one), a puzzle which once defeated 96 per cent of top maths students in America has been republished.
Unearthed by Robbie Gonzalez over on io9, the "string around the rod" test was developed by the International Association for the Evaluation of Educational Achievement (otherwise known as the IEA) to test maths aptitude levels among top level high school students in 16 countries around the world in 1995.
According to Gonzalez, the IEA reported that the puzzle stumped more students than any of the other tests they developed.
Can you solve it?
Answer explained below
As Edward Rothstein points out in the New York Times, the problem requires "almost no advanced mathematics at all" and the relatively "simple" answer can be found using a maths theory that most children are taught in the UK before their GCSEs even begin - Pythagorean.
The easiest way to find the answer is to "flatten out" the rod to give a rectangle measuring 4cm by 12cm.
As Jan De Lange explains in Assessing Mathematical Proficiency: "The string unrolls as a straight line, and it first hits the bottom of the rectangle at a distance 1/4 of the length of the cylinder, or 3cm. The next part of the string starts again at the top, hits the bottom 3cm further along, and so on." Until we are left with this breakdown of four right-angled triangles.
Once the visualisation has been completed, "an application of the Pythagorean theorem" (a² + b² = c²) can be completed to leave us with an answer of... 20cm (e.g. 3² + 4² = 25. The square root of which is 5. Which means the answer is the result of four pieces of string measuring 5cm = 20cm.
More: This is the clearest explanation of when Cheryl's birthday is
More: Here's a quiz you'll probably find even harder than Cheryl's birthday
Top 100
