Multiplying Decimals

[parent2013]

I taught the same method to my dd as 'JustADadHere', ignoring decimals and then adding at the end. Now that she has practiced fraction to decimals and vice versa she says she find it easy to convert them into fractions do the multiplications and then count the zeros in the denominator and add the decimal according to it e.g. two zeros meaning two numbers after the decimal.
Why she find it easier that way is beyond my understanding but whatever floats her boat.

Not sure if that technique will work effectively for decimal fractions and combination of decimal & whole. Try these:

0.05 x 0.36 / 0.006

0.09/0.003

0.008/0.49 x 7

[Guest55]

I am a maths teacher and I am absolutley HORRIFIED by some of the 'advice' on here!

No wonder I have to reteach this when they hit Year 7 ...

Decimal points never move as I said earlier - please do not teach this

[kateJ]

I completely respect that you are a maths teacher, so could you please explain why it is not good to ' move' the decimal point?

[southbucks3]

I am a maths teacher and I am absolutley HORRIFIED by some of the 'advice' on here!

No wonder I have to reteach this when they hit Year 7 ...

Decimal points never move as I said earlier - please do not teach this

Guest 55 sometimes for your own sanity you have to go with the flow....sadly....and I know you are 100% right, but the whole world is just uplifting that little dot and popping it somewhere new in the name of multiplying or dividing decimals.

Prepare yourself...out of fun I just put this in my search engine...You will scream!

http://www.bing.com/search?q=do+decimal ... &sp=-1&sk=" onclick="window.open(this.href);return false;

[mystery]

I teach it to be about the numbers moving e.g. If you multiply by 10 the digits move one place to the left whichever column they started out in. However, I find that children quickly spot for themselves that the decimal point moves the opposite direction - relative motion!

I did groan when my daughter was first of all taught at school to move the point - but if they teach it the other way and then make the connection between the two methods it is good enough for me. It is much easier to "move the point" reliably without losing your place ( excuse the pun).

[berks_mum]

I taught the same method to my dd as 'JustADadHere', ignoring decimals and then adding at the end. Now that she has practiced fraction to decimals and vice versa she says she find it easy to convert them into fractions do the multiplications and then count the zeros in the denominator and add the decimal according to it e.g. two zeros meaning two numbers after the decimal.
Why she find it easier that way is beyond my understanding but whatever floats her boat.

Not sure if that technique will work effectively for decimal fractions and combination of decimal & whole. Try these:

0.05 x 0.36 / 0.006

0.09/0.003

0.008/0.49 x 7

Gave these sums to DD.

For first one she did - 5/100 x 360/6 then cross cancellation and came to 30/10 = 3

Second one - 9/100 ÷ 3/1000 then flip the second fraction (her favourite) and cross cancel.

Third - 8/490 x 7 again cross cancel simplifying 8/70 to 4/35 (asked if i wanted her to go further and convert into decimal. I told her not to bother)

I noted (may be limited to my DCs/ DCs I have observed) -

1) They don't have a fixed method of doing things. They like to vary their methods according to the sums/their moods.
2) They like to find their own methods even if it takes looooong time.
3) Their methods are not always the best/fastest but the chances of them going wrong with their own methods is relatively less.
4) Sometimes I wonder at their methods (read my thickness). Example - DD was doing 3 - 2 4/9 (two four ninths). In a second she said the answer is 5/9. I asked her did you look back in the answers. No mum, you need 5/9 to make this fraction to 3 whole.

Been through the - Why do they have to reinvent the wheel, this child will learn things in a hard way … moments. I guess that is what parenting is about, acceptance more acceptance of the imperfect, swallowing back the urge to improve, infinite patience.

[Proud_Dad]

4) Sometimes I wonder at their methods (read my thickness). Example - DD was doing 3 - 2 4/9 (two four ninths). In a second she said the answer is 5/9. I asked her did you look back in the answers. No mum, you need 5/9 to make this fraction to 3 whole.

That seems the obvious way to do it to me. Out of interest, what different method would you have used then?

[Proud_Dad]

I teach it to be about the numbers moving e.g. If you multiply by 10 the digits move one place to the left whichever column they started out in. However, I find that children quickly spot for themselves that the decimal point moves the opposite direction - relative motion!

I did groan when my daughter was first of all taught at school to move the point - but if they teach it the other way and then make the connection between the two methods it is good enough for me. It is much easier to "move the point" reliably without losing your place ( excuse the pun).

I agree. If you have a sum like 12345.6789 x 0.1 = 1234.56789 you can either think of all nine digits moving one place to the right relative to the decimal point or the decimal point moving one place to the left relative to the numbers. I can see why children might think it simpler to think of a small single dot moving relative to all the numbers rather than trying to imagine all the numbers getting up and shifting one place relative to the dot. So long as you end up with the right answer does it really matter?

I guess its a bit like people talking about the sun rising and setting at the horizon. Most people know that in reality the sun doesn't move and its the rotation of the Earth that produces this effect, but we still all use language which implies the movement of the sun relative to the sky all the same.

[Guest55]

I give up - digits have a place value depending on their position.

The decimal point does not move EVER!!

A '9' in the tens column has the value of 90 - that's why we teach decomposition. If we want a number to be 100 times larger then all the digits (not numbers by the way) need to move 2 places left ie the '9' will be in the '1 000' column representing 9 000.

Sorry this is just not negotiable -

[parent2013]

Gave these sums to DD.

For first one she did - 5/100 x 360/6 then cross cancellation and came to 30/10 = 3

Second one - 9/100 ÷ 3/1000 then flip the second fraction (her favourite) and cross cancel.

Third - 8/490 x 7 again cross cancel simplifying 8/70 to 4/35 (asked if i wanted her to go further and convert into decimal. I told her not to bother)

Well done. That's exactly what I was trying to say i.e. convert decimal into whole. Therefore 0.05 would become 5/100, then cross cancel zeros, multiply/calculate other numbers, then see how many decimal places to move in the end based on zeros left.

Guest55 - To be honest, I'm not actually fussy on that logic of moving numbers or moving decimals as long as the child understands what's underlying logic that decimal applies to the number i.e. times 10 or divide by 10 when a decimal moves one place right or left.

No offence to anyone but I think that there's a serious gap in what state schools teach in Year 4/Year 5 compared to the level expected for top GS. For example see this question below:

9 pens and 5 pencils cost £3.20. 7 pens and 8 pencils cost £2.90.
The unit price for each pencil is _____ and pen is _______.

Parents - any ideas on what school would teach 10 yr old to solve this