430: Dangerous Mistake

What is the more critical mistake, the more damaging to civilization as we know it?

"Me and John are going to the mall" 

or

"1/2 + 1/3 = 1/5"

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 ?

265: Nix The Tricks

Dear Students:

• Which of the following "tricks" was helpful to you in learning and understanding what was going on?
• Which ones just got you confused when the problem changed form slightly?
• Did you ever confuse $\dfrac{3}{5}x\dfrac{1}{5}$ with $\dfrac{3}{5} = \dfrac{x}{5}$ with $\dfrac{3}{5} + \dfrac{1}{4}$?

Which "tricks" should teachers stop using?

260: Annual Factorial

Why don't we need a calculator for this?

Source: UVM, 2011

111: Dirt Triangle

Taken from David Cox, changed just a little:

While I was out, I left a few distance/rate/time problems for students to solve.  Upon my return, I was asking students about the problems and many students had similar responses.
S:  "The Dirt Triangle."
Me:  "?"

S:  "Look, Mr. Cox it's like this...

"...You cover up the one you're looking for and if the other two are next to each other, you multiply.  If one is above the other, you divide."

Me:  "Really? That's strange.  I never learned the Dirt Triangle. I learned ... The Turd Triangle

S₁: "No, that won't work. That's not what he told us."
S₂: "He said it didn't matter how we wrote it."
Me: "So which is it; does one work or are they the same?  Make your case and be ready to defend it."

75: Mixed Up About Mixed Numbers

Say What?

Found this online today. Ack! It's not hard to explain w/real math, people. Why confuse kids? #NixTheTricks @crstn85 pic.twitter.com/EqEHWQeKCx
— Denise Gaskins (@letsplaymath) March 13, 2014