Tuesday, October 21, 2014

287: Fahrenheit to Celsius

We all know the conversion from °C to °F: multiply by 9/5 (or 1.8, if you can't handle those evil fractions) and then add 32.

The conversion from °F to °C is more complex: subtract 32 and then multiply by 5/9 (or 0.555555555).

That's cumbersome for mental math.

Here's a shortcut: subtract 30 and divide by 2 OR multiply by 2 and add 30.

  • How good is that shortcut?
  • Are there temperatures that it's okay for and other temperatures that the shortcut is too far off?
  • Is this just another stupid shortcut? NixTheTricks ?

Monday, October 20, 2014

286: Sphereometer

How does this thing work to find curvature?


What kinds of information do you need to make the markings on that dial?

Sunday, October 19, 2014

285: Three Angles Revisited

Two days ago, I posted Okay for a Fifth-Grader?

Five Triangles said, "We posed the question slightly differently, our diagram providing an important hint to bring it to a more manageable level for younger problem solvers."


What do you suppose the "important hint" was?


Saturday, October 18, 2014

284: Analyzing Digits


When the integers from 1 to 30 are multiplied, determine how many consecutive digits starting from the ones (1s) position are zeros.
Why does this question not require a calculator?

Would this be a fair question for some standardized test?

source.

Friday, October 17, 2014

283: Ok for a Fifth-Grader?

Three squares of equal but unknown size.


Is this a fair question to ask a fifth-grader?


From a Numberphile video.

Thursday, October 16, 2014

282: Number puzzle


"The sum of two positive integers is 216. The greatest common factor of the two numbers is 24. What are all the possible pairs of numbers?"

What approach seems the easiest here?
I can see using solution methods such as:
  • algebra
  • guess and check
  • organized list
  • visual representation
Which (or which other) method rings true for you?
Which of the two sentences eliminates the most numbers?