Multiplying by 10,100,100 with decimal points

Discussion in 'Mathematics' started by Ms_Saba, Jan 19, 2012.

1. Ms_Saba

Hi,

I'm a trainee and have been observing my Year 5 maths (set B) class struggling with doing:

6.7 x 10,
568 /100

etc.

We are not allowed to move the decimal point, although that is how I was taught many years ago. The kids have had numerous lessons on it and don't get it. The teacher has asked me to bring a fresh approach to it.
I have seen the slider cards but am hoping to get some more resources from you pros out there.

I would love to know practical methods you have used to get this concept through.

Cheers

S

2. Ms_Saba

Hi,

I'm a trainee and have been observing my Year 5 maths (set B) class struggling with doing:

6.7 x 10,
568 /100

etc.

We are not allowed to move the decimal point, although that is how I was taught many years ago. The kids have had numerous lessons on it and don't get it. The teacher has asked me to bring a fresh approach to it.
I have seen the slider cards but am hoping to get some more resources from you pros out there.

I would love to know practical methods you have used to get this concept through.

Cheers

S

3. bobboots

And there lies the problem. You were taught that way and can do it.
They were taught a different way and can't.
I'd move the point when noone is looking.

4. bombaysapphireStar commenter

I use the slider cards.
Laminate some pieces of squared paper and cut lengthways into 4 long strips. Take a piece of card for each student. Cut a pair of slits close to the middle, width approx 1cm.
Depending on ability I get them to draw the place value grid on with a ruler, or do one myself and photocopy it on to the card so they can just enter the headings. How long it takes for a class to get it depends on ability but they all get there eventually.
I was teaching decimals to fractions to a bottom set Year 10 today. They have used the sliders, and during a discussion on place value, one pointed out the link to sliding the digits when multiplying by 10

5. oakapple

I love the slider cards too! This is good to show that the numbers stay together and don't suddenly split up and become eg 2007 for two hundred and seven. I've been using my grids now for over 20 years - wonderful inset day when I was shown this idea!!
I also use the place value headings M HTh TTh Th H T U decimal point t h th etc etc
Write them at the top of the columns and use squared paper as its easier to write the figures in the right places. Emphasise that the figures move, not the dp.
I also use digit cards and place values headings on a sheet of paper so the children can move the figures... again.
Have also added labels to chairs and the children have sat in the seat holding a number then physically moved to a different position.
I always talk about making the number ten times more, one hundred times more etc.

6. carriecat10Established commenter Community helper

In my experience when children can't multiply or divide decimals by 10, 100 etc they need practical apparatus such as Dienes/base 10 to enable them to manipulate the resources and get a real sense of what happens to numbers.
This can be used alongside such things as slider cards, but these alone don't really help to explain the relationship between the numbers.
I would also suggest approximating the answer first as this will help children recognise when they have made simple errors.
Carrie

7. DeborahCarolNew commenter

Yes, the problem with 'move the digits' is that often the students DO need 'numerous lessons on it and don't get it'. Slider cards, place value charts...., sure, but in a SAT test they won't have those to hand! All an object lesson in making something that was very easy for us very hard for modern children.
However, if you show them how the digits move, reminding them that the decimal point doesn't really move, and THEN say...here's a little trick to do these multiplications and divisions easily and quickly (yes - 'move the point'!), they can do it after ONE lesson.
However, you will probably be thrown off your PGCE course if you dare to commit such a heresy.
I won't comment further, as I raised this 'point' (!) a couple of months ago and the thread attracted many posts! From memory, primary teachers were adamant that 'move the point' should not be taught, but at least half of secondary teachers 'admitted' to teaching 'move the point' (the method used by most maths graduates, most of America, most of Asia, etc).
http://community.tes.co.uk/forums/p/534353/7151799.aspx#7151799

8. chris reall

When we are told you 'mustn't' teach in a certain way, I have found it useful to do a search to find out if other countries have the same ban. A brief search will show you that the moving of the decimal point is still alive and kicking elsewhere.

I find a piece of blu-tack in place of decimal point works really well on board for demonstration purposes. I tell them to count the zeros, and move the same number of spaces to the right for multiplication, or to the left for division, putting in extra zeros if necessary. If I teach it as a mantra, they soon get it.

And yes, I know that they may not understand why they are doing this, but I really don't think that's important unless they are going to be mathematicians. Most children just need to have the practical skills to convert measures in real life.

All very non PC, of course.

www.reall-languages.com

9. carriecat10Established commenter Community helper

Couldn't disagree more!
Surely we are aiming to help children understand otherwise how will they be able to use and applying these skills in any real-life contexts? Aren't you contradicting yourself by then saying they just need practical skills to convert in real life?
What will happen to those children who struggle to remember the 'trick'?
Carrie

10. PaulDGOccasional commenter

Well, at the moment, they're turning up in secondary school unable to work out how to do it "properly" or to use the trick.

And, while it would be nice for them to understand, it would be better for them to be able to at least do!

If they could do, they might find that eventually they "cotton on" to why the method works, but even if they don't even do that, at least they leave with a useful skill.

At the moment, far too many neither "understand" nor can "do".

(And frankly, everyone I know who actually does manipulation of decimal numbers on paper moves the point - moving the numbers is far too much work and good mathematician never looks for extra work.)

11. sparkleghirlStar commenter

As a young, fresh-faced teacher I was suprised to find you weren't suppsoed to move the point. It was one of those things I was too embarrassed to comment on at the time, afraid of being laughed at or ostracised (or failed).
Now, with more years experience under my belt than I care to count, I realise the absurdity of persisting with a method which makes life so much harder for the children.
Fortunately, being in secondary, I can just do something like 'so I bet you were told you had to move the numbers not the point? Well here's a trick that'll get you the same answer....'
When we're following dogma at the expense of children's learning si when you realise thos in charge have lost the plot.

12. CB123New commenter

I dont understand how moving the point is easier than moving the digits - surely both are as easy / hard as each other. Guess it is important that only 1 thing that moves as if the children are hearing move the digits, then move the decimal point and thus do both then they will get in trouble.
Personally (and yes im primary) I think referring the numbers back to place value and children understading that the decimal point is always next to the unit is important therefore it cant move
Teaching the children that x 10 moves the numbers up to the next place value column is giving them more of an understanding.
As for an activity, I have used the moving digits ITP, also I often get the children being the numbers and them actually jumping (left more multiplication, right for division) We have also learnt a song from the number fun set of books but I know singing is not everyones cup of tea

13. PaulDGOccasional commenter

I don't think this is something anyone had any difficulties understanding until someone decided that teaching "moving the point" wasn't to be allowed.

The insistence that children must "understand" instead of children must "do" is at the root of current problems in maths - get them doing when they're young and can cope with that. Get them understanding when they're older and have developed enough to cope with it.

14. AnonymousNew commenter

Hear hear!! I employ a number of Maths graduates (who mainly teach secondary) and that is exactly their approach. Just move the point, it is a simple trick that works! I also have experience in an internationsl setting (particularly Australia and India). Interestingly, they also move the decimal point!
Why do teachers so insist on over-complicating everything?

15. Polecat

I agree absolutely with PaulDG and etyke.
The history of mathematics is littered with examples of things that worked without anyone understanding them until much later: Pythagoras' theorem; irrational numbers; complex numbers; calculus; asymptotic series; renormalisation ....

16. AnonymousNew commenter

Interestingly, if you look at some very good GCSE support materials from Mathsclip, they use the principle of moving the decimal point.