Increase and decrease

Hello, and welcome to Myriora Maths! A forty pound coat is in a sale, with twenty per cent off. How much does it cost now? And what happens when a price goes up by ten per cent? Let's see.

First, find twenty per cent of forty pounds. Ten per cent is four pounds, so twenty per cent is eight pounds. That's the discount. Take it away: forty pounds take away eight pounds is thirty-two pounds.

Prices can go up too. A fifty pound ticket goes up by ten per cent. Ten per cent of fifty pounds is five pounds. Add it on: fifty-five pounds!

There's a quicker way. Twenty per cent off leaves eighty per cent of the price. Eighty per cent of forty pounds is eight lots of four pounds: thirty-two pounds. Same answer, in one step!

Let's try one together. A thirty pound game has ten per cent off. What does it cost now? Say it out loud!

Ten per cent of thirty pounds is three pounds. Thirty take away three is twenty-seven pounds!

Here's a question that catches lots of people. A price goes up by ten per cent, then down by ten per cent. Is it back where it started? Some people say yes. But try a hundred pounds. Up ten per cent is a hundred and ten pounds. Then ten per cent of that is eleven pounds, so down ten per cent is ninety-nine pounds!

Your turn! A sixty pound bike has twenty-five per cent off. What does it cost now? Pick the right card. You've got twenty seconds, or pause the video. Go!

Time's up! Twenty-five per cent is a quarter. A quarter of sixty pounds is fifteen pounds. Sixty take away fifteen is forty-five pounds!

Quick review. For a decrease, find the percentage and take it away. For an increase, find the percentage and add it on. Or do it in one step: twenty per cent off leaves eighty per cent. And ten per cent up, then ten per cent down, does not get you back! See you next time!