Addition & Subtraction


Posted by Miss Crowther | Posted in Uncategorized | Posted on March 17, 2015

In numeracy, we are learning more about addition and subtraction. We have been working with whole numbers, common fractions and decimal fractions.

Last week we focused on solving problems using equivalent number sentences. Here are some examples of equivalent number sentences:

14 + 17 = 15 + 16


385 – 60 = 390 – 65


(3 x 3) + 11 = 4 x 5

The number one thing to remember when working with equivalent number sentences is that the answer on each side of the = has to be the same.
Different strategies can be used to work out any missing numbers.

181 + 7 = 185 + ?

For the example above, we discovered that if one of the numbers in a problem gets larger or smaller the other number does the opposite. For example, 181 got larger by 4 to become 185. So, 7 has to get smaller by 4. Therefore, the ? = 3. You can check it by doing 181 + 7 = 1881 and 185 + 3 = 188 too. It works because each side has the same total, just with different numbers.

73 – 9 = 75 – ?

When you solve subtraction problems, you have to make sure the ‘gap’ or difference between the numbers stays the same.  Both numbers have to increase by the same amount. So, ? = 11.

Click HERE to do some of the activities we did online to practise working with equivalent number sentences.

This week we are adding and subtracting fractions. Before we started, we shared all the different ways we had already used fractions in the last 24 hours. It was surprising to find out how many ways we had used a fraction of something – from 1/2 a cup of milk with our porridge to eating one sixth of a birthday cake.

We know that the denominators (the bottom part of the fraction) have to be the same before you can add or subtract. This is because the denominator tells you if the fraction is a half, third, quarter and so on.


2 + 5 = 7

8    8    8


 Click HERE to practise adding and subtracting fractions in an online game.

Sometimes you have to change the denominator if they are not the same to start with. We have looked at how to find the lowest common denominator.

Can you use an equivalent number sentence to solve the following problem?

When this number is added to 13 the answer is the same as 10 + 9. What is the missing number?


When do you add and subtract fractions in Real Life?


Have you got an addition or subtraction problem for us to solve? Please share it!



Comments (14)

Hi class,

Working out that missing number…. I think the answer is 6. The way I worked this out is…..
First I worked out 10 + 9 (the answer is 19.) Then I had to work out what plus 13 equals 19 (which was 6.) So that’s my answer. Do you think I’m right?

I don’t think I add and subtract fractions in real life but I definitely use them. Same with percentages and decimals.

My problem to solve is……
I am two times bigger then 125, take away 95 what number am I?

Happy Blogging

From Bailey

Hello Bailey,

I agree with the missing number, I also think it is 6. My workings out were quite similar but I’ll show you anyway.
10 + 9 =19
19 – 13 = 6!
That’s how I got my answer and your way works perfectly as well!

I don’t use fractions in real life either, well not that I know of. I usually use decimals or mental algebra to work numbers (money, etc) out.

I think your problem works out to 155. Am I right?

My problem for you is….
If you take away 10 from me, times me by 2 and the answer is 300 what number am I? NO CHEATING WITH A CALCULATOR, USE STRATEGIES!

Ok, I got to go,

Hi Tayla,
Thanks for replying to my comment 🙂
Yes you are right! The answer is 155! Thanks for working it out!
Working out your problem……..
I think the answer is 160. I think this because
160 – 10 = 150
I50 x 2 = 300. So, am I right?
Happy Blogging 🙂
From Bailey

Hi everyone,

Looks like you are all enjoying maths and learning all sorts of new things. At our high school most of our work is pretty easy because UPPS was so good I already know half the stuff they’re teaching us!

For the first question I think the missing number is 6.

As most of you know, I am fascinated with food so from my experiences you would use adding and subtracting fractions when you have 3 older brothers and we’re all fighting over who gets the last piece of pizza.

My maths problem is:
I am greater than 5 but smaller than 10 and when you add 57 to me you get 64. What number am I?

Bye for now

Hi Alicia,

It is wonderful to hear from you! Thank you for visiting the blog and leaving a comment. 🙂

I can imagine that things could get a little competitive when you are divvying up fractions of pizza with your older brothers!

I think your number is seven. Do you want to know my strategy?! I shall share it anyway. I turned your word problem into an equation; 57 + y = 64. Then I thought, 64 – 57 = 7. I checked it by working out 57 + 7 = 64. Am I right?

I hope you are having a ball at high school!

Take care,
Miss Crowther 🙂

Hello Miss Crowther,

How are you? 🙂 Do you have plans on the weekend or what do you think you are going to do on the weekend? 🙂

I have a word problem for you to solve without using a calculator.

Emma has 120 sets of Lego friends she still needs to build. Her friends are having 85 parties. They have one party each. Emma gives her 85 friends 1 packet each. Then Emma steals back from some of her friends and took 28 packets that she gave out to her friends back home. How many packets of Lego friends does Emma have NOW?

Have an AWESOME weekend! 🙂 🙂 🙂

Good luck solving my word problem. 🙂 🙂 🙂 🙂 🙂

From one of your students,
Becky. 🙂

Hi Becky,

I’m well, thank you. How are you?

I am not sure what I will do over the weekend. However, one thing I know for sure is that I will be trying to have a sleep in!! 🙂

If Emma gave away 85 of her 120 Lego friends, she would have 35 Lego friends. If she then took 28 packets back she would have 35 + 28, which equals 63. Am I right?

I hope you have an awesome weekend too!

Bye for now,
Miss Crowther 🙂

Hi Alicia,

Thanks for commenting on our blog!

I think the answer to your problem is 7. I worked it out in my head.

Got to go,
From Bailey

Hi guys!

I love equivalent sentences, they are fun to work out.

I think the answer to your first question is 6.

I do a lot of adding and subtracting of fraction in real life. When I order the pizza every Friday night I have to add up how many eighths of the pizza everyone is eating and then I know whether to order one, two or sometimes three pizzas. I also talk about equivalent fractions then. I’ll say “Jamie, your half of the pizza is on that plate, there’s four pieces”, “I’ll have two slices so that’s my quarter”, “Harley, you have half of the Aussie pizza and a quarter of the meat lovers. You’ve got six slices.”

Have fun!

Mrs Laffan

Hello Everyone,
It’s Kaitlyn here, I love maths!

10 + 9 = 13 + ____ The missing number is 6.

Often if you’re cutting a cake, have pizza or biscuits and you want to share them out evenly you will use fractions. For Example, If there are 10 slices of cake and you are sharing them with 4 friends (Remember to have some yourself 🙂 ) You would each be able to eat two slices. Then you will have shared them out evenly.

What is your favourite part of maths?

Happy Blogging 🙂 🙂 🙂 🙂

From Kaitlyn

Now my problem for you to solve is: All of the pizzas have ten slices and are all the same topping. I ate 2 pieces, one of my friends ate 5 pieces because they were really hungry and my other friend ate 4 pieces because they were hungrier than me but not quite as hungry and my first friend. In total did we eat two pizzas? How did you work it out?

Hello Kaitlyn,

I also love maths and I think we both know that!

I got the same answer as you! I didn’t think about fractions when cutting up food!

Next time I think about fractions I will use food in an equation.

My favourite part about maths is when we get to do algebra and formal algorithms. What’s yours?

I think the answer to your problem is 11 slices which isn’t 2 pizzas. I figured this out by…..
you ate 2, one friend ate 5, one friend ate 4
4 + 2 + 5 = 11

There is one cake with 13 slices. One person eats 5 slices and 3 people ate 2 slices, how many slices are left?

Got to go,

Hi 5/6 Team,

This week we have been learning about chance. We collected data from the class and then made different graphs on Excel. We made pie graphs and bar graphs and much more. Have you done chance yet?

From Harry, Lachlan and Kristin

Dear Miss Crowther,

Thank you for all the helpful information i will use it and i hope to see ya’ll post again soon.

From,Natalie H

Hi Natalie,

Thank you for visiting our blog. I am glad you found the information helpful!

Miss Crowther 🙂

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