Witspeak Podcasts
Log in Start a podcast
Chalk Dust and Common Sense artwork
Chalk Dust and Common SenseS1 · E10:55176 plays Members only

Percentages, and why 20 percent off then 20 percent on is not zero

0:00 / 0:55

This episode is for members. Sign up free or log in to hear the whole episode.

RSS
Show notes

Percent of what

The first episode tackles the question Ciara hears most from adult learners: why percentages seem to behave differently depending on the direction you go. She works through a shop discount, a VAT calculation and a pay rise that got reversed, using only numbers you can hold in your head.

  • The one question to ask every time: percent of what
  • Why a 20 percent cut followed by a 20 percent rise leaves you short
  • A mental shortcut for 10, 5 and 1 percent
  • Reading a news headline about a 300 percent increase

A listener question about tips and service charges closes the episode.

Transcript
Hello and welcome to Chalk Dust and Common Sense. I am Ciara Mulvaney, I taught maths for nine years, and I now spend my days with adults who were told they could not do it. Today we start with percentages, because they are everywhere and because they cost people real money. Here is the puzzle. A jacket is 100 euro. The shop takes 20 percent off. Then the sale ends and they put 20 percent back on. Is the jacket 100 euro again? Most people say yes. It is not, and once you see why, you will never be fooled by a headline or a discount again. I will give you a few tricks for doing this in your head, and at the end a listener asks about service charges on restaurant bills. No calculator needed, just your ears. You are listening on Witspeak Podcasts.
Open as text

More from Chalk Dust and Common Sense