## You Reckon?: General Teaching Notes

**Introduction **

When will the world's oil reserves run out?

How much land would be needed to build a wind farm to supply 25% of London's electricity?

If the sea level rises by 50 cm, how many people will be homeless in Bangladesh? London?

What will be the population of Europe in 2020?

An earthquake destroys Istanbul - how many people are homeless, how much water, food and housing is needed, and how long will it take you to get it all there?

What would it cost to give every pensioner another £10 per week?

Can I afford to run a car?

"You Reckon?" introduces the idea of *Plausible Estimation (PE). *PE underpins major decisions we make in our private and public lives. PE tasks show pupils just how powerful simple mathematics can be when making decisions about important topics. Plausible Estimation is also great for debunking spurious claims ('we can grow all the biodiesel we need on set aside land').

Plausible Estimation is an important part of mathematical reasoning - problems have to broken down into simpler components; multiple approaches are rewarded; and pupils have to communicate well. PE rewards technical mastery, and reinforces the need for mathematical fluency when doing 'back of an envelope' calculations. PE encourages pupils to make mathematical thinking an integral part of their common sense. Pupils will be able to see that the problems they are being asked to solve are the same problems faced by aid agencies, government planning agencies, and salesmen!