Percentages, and why 20 percent off then 20 percent on is not zero
Ciara starts with the number trick that costs people money every day. Percentages of what, exactly, and how to check a sale, a tax and a pay rise in your head without panicking.
Why algebra uses letters, with a carpenter who never calls it algebra
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Show notes
The letter is just a box
Algebra is the moment most people checked out of maths, usually because nobody explained why a number suddenly became an x. Ciara brings in carpenter Donal Fitzgibbon, who has been solving for unknowns on building sites for 30 years without once calling it that.
- What x really means: a box with something in it
- Donal works out a staircase rise from the room height, out loud
- Rearranging a formula is just undoing steps in reverse
- Why school taught the method before the reason
Try the stair problem yourself before Donal gives the answer, then see where you went wrong.
Transcript
Welcome back to Chalk Dust and Common Sense, I am Ciara Mulvaney. This week we go to the place where most people's relationship with maths quietly ended: algebra. One day it was numbers, the next day there were letters, and nobody told you why. I have a guest who can explain it better than any textbook. Donal Fitzgibbon has been a carpenter in Limerick for 30 years. He builds staircases, and every staircase involves an unknown he has to work out from what he already knows. He does it on a scrap of plywood with a pencil. He has never once called it algebra. Listen to him work through a real job and I think the letters will finally make sense. Pause and try the staircase yourself when I tell you to. You are listening on Witspeak Podcasts.
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