Category Archives: Problem Solving

205. Percentages cubed

This neat little activity combines nets of cubes with non-calculator percentages. It doesn’t necessarily replace teaching basic percentages, but it is a good starting point.

Objectives

  • To understand the link between different percentages.
  • To construct a cube, using a 2D net.
  • To calculate percentages.

Equipment
Pre-printed nets of cubes on card
Scissors
Glue
Felt-tip pens

Task 1
Cut out the net of the cube and mark the midpoint of each edge, ensuring matching points on the joins.

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The matching points are important later on. Fold in both directions along each line.

Task 2
Label the middle square 100%.
Draw horizontal and vertical arrows going away from it with divide by 2, 4, 10 & 100. What percentages should go on these squares?

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You will notice the arrows go through the midpoints.

Task 3
After your class have identified the squares as 50%, 25%, 10% & 1%, ask your class how these are linked.

This bit wowed my class.

Draw a vertical line up from 50%.
Label it divide by 2.
Draw an arrow coming in from the left of the 25%.

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What happens if you fold the cube up?
The arrow joins up! This is why you need the midpoints.

Task 4
Ask your class to complete the labels linking the other percentages.

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Task 5
You should have one empty square. Label this 5%.

Ask your pupils to complete their arrows. They can be completed with divide or multiply.

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You will notice that mine is colour coded, based on the original percentage in each calculation.

Task 6
Glue the 5% square flap to the 25% square. This allows you to temporarily tuck in the other flaps, whilst allowing the cube to be folded flat to go in a book.

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Task 7
Give pupils a starting number eg 360. This represents 100%.
By following the arrows on the cube, they can work out all these percentages quickly and efficiently.

204. Revolution in Volume

Most elements of Core Maths can be visualised with a good diagram, but volume of revolution can be tricky if your technical drawing skills leave something to be desired. My colleague JA came up with a visualisation which is simple and elegant, yet also fun and memorable.

Step 1
Start with a curve. Introduce the limits a and b. Discuss what shape a thin strip would make: a disc.

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Step 2
What would several discs make?
Now this is the cool bit:

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This innocent looking shape is a pop up gift tag:

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You can demonstrate what happens if the curve rotates 180 degrees around the x-axis.

Step 3
Now the really fun bit: dig out those interesting honeycomb christmas decorations, a metre stick and some tape:
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The metre stick represents the scale on the x axis. The decoration represents the full 360 degree revolution about the axis.

Hint
Since these decorations are made from paper and card. You can use a sturdy craft knife to cut them into other curves. They also make great wall displays.

203. Sunny Surd Sunflower

Today we have a guest contributor to the Sandpit – my colleague BH.

His Year 9 class have been studying Surds. They have just completed two particularly difficult Tarsia puzzles on simplifying surds. He celebrated their success by getting the class to create a ‘Sunny Surd Sunflower’ – what a great way to celebrate springtime and achievement.

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UPDATE: Inspired by the bright sunflower, my class created an AVERAGE caterpillar!

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201. BBC Crispies

There was an interesting discussion on the BBC Breakfast programme this morning about the exchange of maths teaching ideas between British and Chinese teachers.

The guests on the sofa were from the NCETM and a serving Head of Maths. There was mention of the innovative ideas used to teach Maths in Britain – including some of mine. I’m not being presumptive, I happen to know that Head of Maths – in fact some of his ideas are on this site (JDs Tree Diagrams). So just in case you missed Breakfast, here is some Cake.

 

197. £40.95

Today we have a discussion starter question for you, inspired by a trip to the shops.

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My shopping cost £40.95 today. What is the smallest number of coins required to make this amount?
If I paid with two £20s and a £10 note, what is the most efficient change?
Why would someone pay £41.05, as opposed to £41?

I purchased 17 items, do you have enough information to calculate the mean?
The most expensive item was £10, the cheapest was 45p. What does this allow you to calculate?
Two luxury items cost £9 in total. If I hadn’t bought these, what would the mean have been? Does this effect the range?

When I paid I was given this voucher:

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What would the shopping have cost somewhere else?
What would the mean cost per item be after this discount?
What percentage discount is this?

You could also use this as a discussion starter about the number skills you use when you go shopping.

195. Marshmallow Maths

It’s our first birthday at the MathsSandpit and this post is party themed. Remember a few years ago, when chocolate fountains were the ‘in thing’ at celebrations and parties. The healthy guests stuck to strawberries drenched in chocolate. The unhealthy went for marshmallows on sticks and … well … all I’ll say is Geraldine Granger (Vicar of Dibley – Chocolate Fountain)

I’m trying to decompartmentalise the maths in my students heads. They struggle to see the links between different topics. So I introduced ‘Marshmalllow Maths’ – they were intrigued/hungry as soon as I mentioned it.

Equipment

  • Cocktail sticks
  • Pink and white marshmallows

Step 1

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Ask your class to connect the marshmallows together

 

Step 2

What mathematical characteristics do the marshmallows have? I’ve summarised my classes’ responses below:

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Two marshmallows lead to ratio, percentages, fractions, decimals and probability. The links between these topics start to emerge.

Step 3

Add on another marshmallow

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How have the ratios, fractions, decimals, percentages changed?

 

Step 4

Make another 1:2 ratio marshmallow, identical to the previous one. How have the mathematical facts changed? In fact although the numbers have changed, the proportions have stayed the same which is proved when you simplify the numbers. Physically you can prove it by stacking the structures on top of each other – from above it looks like the original structure.

At this point I went cross-curricular and discussed the similarities between the marshmallow structure and water (H20). I was going to label the marshmallows with H and O, but my food-colouring pen wasn’t working. My logic was that water always has hydrogen and oxygen in the same ratio – this means we know we can drink it. If the ratio suddenly changed to H2O2, we would be in trouble! As far as I can remember H2O2 is hydrogen peroxide and is better for bleaching than drinking. This actually got the idea across quite well – no-one tried to fudge their ratios.

 

Step 5

I then allowed the class to make their own simple structures using their own piles of marshmallows. They had to make at least three identical structures, work out the related maths and prove that their numbers could be simplified to the basic form. In doing so they also looked at converting ratios to fractions and also found fractions of amounts.

 

Step 6

Eat marshmallows (whilst doing some related questions).

 

Optional: Step 7

Calculate the percentage increase in body mass on results day! It was marshmallows today, a chocolate prize for cracking a code earlier in the week and they say they learn better when they eat. I think it’s all a ploy to scrounge more food … but if it works … maybe fruit next time!

191. Fallen Revision

Which Maths teachers out there are fed up of stressing the same basic exam/test skills? Come on, there must be more than that? You there at the back. That’s more like it!

Unfortunately, us teachers don’t understand student basics:

* Pencils are for chewing, flicking or breaking.
* Rulers are for poking and twanging
* Working out is detrimental to doodling time
* And as for Units – wasn’t that mentioned in PSCHE to do with alcohol?

Sound familiar?

This term I’ve made my class reflect on the basics using a ‘Fallen Phrase’ puzzle template from Discovery Puzzlemaker. The skeleton of the phrase is given, but the missing letters are stacked at the bottom of each column – a bit like a collapsed ‘Wheel of fortune’ puzzle.

Wheel of fortune

The puzzle covers all the basic skills, but it is difficult. My students had to really think what I nag them about, rather than just rearrange the letters.

I just hope all their hard work pays off in their test.

Revision Hints fallen phrase.

Visit the Discovery puzzlemaker site.