July, 2009 | Maths Blog

Coming Soon: Mental arithmetic and written subtraction

Holiday time is upon us now, although the promised hot summer has not materialised yet! To pass away a few idle moments, or to keep the brain in trim ready for next term we have both mental and written methods of calculating coming next week.

Mental arithmetic is the core of number work in the primary school as later written methods depend entirely upon successful mental methods. Next week we have page on how to add 19 to any 2-digit number and a great little page for Year 2 children on missing numbers on a number square. In our Four Rules section we will be looking at subtraction of money, including the decimal point, using the standard written method of subtraction.

Amazing date: August 7th 2009: 040506070809 or 123456789

Two truly amazing times will shortly be with us – but only for a second each.

At 5 minutes and six seconds after 4 am the time will be o4:05:06.

On August 7th the date will be 07/08/09.

Put these two together, so at 5 minutes and six seconds after 4 am on August 7th 2009 the time and date will be:

04 05 06 07 08 09

This will not happen again so enjoy the moment.

Perhaps even better a few hours later the time will be 12 hr 34 minutes and 56 seconds or 12:34:56, so at 12:34 and 56 seconds on 7th August 2009 the date will be:

12 34 56 7 8 9

A unique time to be enjoyed by all!!

(As usual the Americans have jumped the gun as they show the month before the day but we all know this is the right way to do it!).

Year 3 Maths worksheet: Money problems

This is the first of a new series of maths worksheets for Year 3 on money and shopping problems. This page looks at addition of whole tens at the greengrocers, working with multiples of 10p.

These questions should be solved mentally, although children may want to make jottings to help them. Most only need one operation to work out although the later questions involve both addition and subtraction.

Money: shopping with pence (pg 1)

Resource of the Week: Counting games for early years

We have a great set of fun maths games for Reception/Early Years and one of my particular favourites is the Counting Goats game. This is really good practice at counting up to 5. Young children can not get too much practice with this both in the home and on the computer. They may well use their fingers to help and count out loud, but after a time they will begin to count in their heads. Adults can often glimpse at a picture to see how many there are, almost without counting – don’t expect this of 5 a year old!

Don’t forget to click on the banjo playing goat at the end for a happy tune!

Counting up to 5 game

Mental arithmetic: Subtract 9 mentally

Here we have a straightforward mental arithmetic worksheet on subtracting 9 from any 2-digit number. Usually the best way to do this is to subtract ten and then add one. Probably the only time that this is not the best way is if the number to take 9 from also has a 9 in the units.

Knowing this quick technique gives children confidence and they can then move on to subtracting 19, 29, 39….. etc in their heads.

Subtract 9 from 2-digits

Mental arithmetic: Know by Heart Pairs that Make 20

This maths worksheet for year 2 children looks at pairs of numbers which make 20. Before doing this children should be confident with, and know by heart, all pairs of numbers which make 10. Once they know these, pairs that make 20 becomes easy; just one of each pair is ten more e.g. 6 and 4: 16 and 4. Being able to work quickly with mental arithmetic is one of the keys to success in school as so much of the maths lesson today is carried out orally and children gain or lose confidence very quickly when answering in front of a whole class.

Know pairs that make 20 (pg 1)

Maths Worksheet: Decimals with a total of 10

Another mental arithmetic maths worksheet on decimals, suitable for Year 5. This page looks at pairs of decimals that make 10. Once again there are several ways of tackling these questions.

One way is to add on from the smaller number, firstly, the tenths to make the next whole number, and then the units to make 10.

eg 10 – 6.7

Count on from 6.7 to 7 which is 0.3

Then count on from 7 to 10 which is 3

Answer 3 + 0.3 = 3.3

Another way is to count on the units first and then the tenths, but beware the answer which counts on 4 from 6 to make 10 and than counts on 0.3 as this will result in an answer which is one too many: a common slip-up.

Another way of doing this is to ignore the decimal and think of the question as 100 – 67. Work it out mentally (which is 33) and then convert back to 3.3.

Decimals with a total of 10

Maths Worksheet: Add 9 Mentally

Being successful with mental arithmetic is all about having a ‘feel for numbers’ and being able to manipulate them to suit the way you like to calculate. Knowing a few basic ‘tricks’ helps enormously with this and gives children confidence.

Adding 9 might sound a little dull, but knowing that you can do it in your head by adding ten and subtracting one can make all sorts of other mental additions easy, as we will see later adding 19, 29, 39 etc all follow the same path as do adding 18, 28, 38 etc; these tasks which at first glance might seem tricky end up being easy.

This is suited to year 3 children who are confident with adding single digits and can count up to 100.

Add 9 to 2-digit numbers

Maths Worksheet: Counting up, crossing thousands

There are many occasions when a 4-digit subtraction can be done ‘in your head’. These questions, suitable for Year 5, are examples of this. They all involve numbers which are just over and just under a whole thousand.

For example: 3003 – 2994

Probably the easiest way to do this mentally is to count on 7 from 2993 to make 3000 and then count on, or add, the extra 4, making 11.

This is much easier than doing the question on paper, with lots of ‘borrowing’ and carrying, crossing out etc!

Count up crossing thousands

Coming soon: Counting, addition and more decimals

You can really hit the mark with next week’s selection of worksheets. Most will be to do with mental methods of counting and addition. Firstly, we have counting on with large numbers. Most of these questions cross the thousands boundary and also provide good practice at reading larger numbers. For Year 3 we have an addition worksheet on how to to add 9 to 2-digit numbers mentally. There will also be a page of standard written addition of 4 digit numbers in our Four Rules section.

For older children there is a page on decimals, working out decimals with a total of ten.

If you are looking for addition worksheets, don’t forget to look in the Four Rules section as well as in each individual year under Knowing Number Facts and Calculating.