Equal Additions

I am learning to solve subtraction problems by equal additions that turn one of the numbers into a tidy number.

Numbers like 10, 20, 30, 40 etc.

Alice has $304 and spends $198. How much money does she have left?

Step 1: We are trying to find the difference between these two numbers so let’s imagine them on a number line.

$304 - $198

$304$198

Step 2: If we move $198 up to $200 this question would be much easier to answer. Because we are moving one end up by 2 we must move the other end up by 2 to keep the distance between the two numbers the same.

$200 $306+2 +2

Note how the distance between the two numbers hasn’t changed so the answer will be the same.

Step 3: 306 – 200 = 106

$304 - $198 = ….

+2+2

Below is the same problem but this time I have shown how I like to record my working. You might like to do this too until you are ready to do the whole process in your head.

$306 - $200 = …. 106

Both sides are getting bigger by 2. I do the same to both sides so I can keep the gap the same.

I chose to increase $198 to $200 to make this question easier to answer.

Let’s see what happens if I change the first number into a tidy number.

$304 - $198 = ….

-4-4

$300 - $194 = …. 106

Since I subtracted 4 from one side I had to subtract 4 from the other side.

Notice how this question gets the same answer but it is harder to work out.

Always change the second number. Not the first

724 - 299 = ….

+1+1

Let’s try this example together. Try to do each step in your head before clicking.

…… - …… = …. 106

724 – 299 = ?

Step 1: Which number will you change and what tidy number will you change it into?

300

Step 2: You have changed 299 into a tidy number by adding 1. What do you need to do to the other number?

725

Step 3: What is the answer?

Now try these questions on your own.

Remember to first change the question then answer.

Need some more practise? Try Pg52 of Teacher Tools Addition and Subtraction Book Numeracy resources

1) 237 – 198 = ?

2) 435 – 297 = ?

3) 684 – 499 = ?

4) 514 – 398 = ?

5) 317 – 97 = ?

Click again when you have completed each question and the answer will be displayed.

239 - 200 = 39

438 + 300 = 138

685 + 500 = 185

516 + 400 = 116

320 + 100 = 220