Transcript Equal-Additions

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? $304 - $198
Step 1: We are trying to find the difference between these two
numbers so let’s imagine them on a number line.
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.
Note how the distance
between the two
Step 3: 306 – 200 = 106
numbers hasn’t
changed so the answer
will be the same.
$200
$198
+2
$304 $306
+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.
Let’s see what happens if I change
the first number into a tidy number.
$304 - $198 = ….
$304 - $198 = ….
+2
+2
-4
-4
$306 - $200 = 106
….
$300 - $194 = 106
….
I chose to increase $198 to $200 to
make this question easier to answer.
Both sides are getting bigger by 2. I
do the same to both sides so I can
keep the gap the same.
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
Let’s try this example together. Try to do each step in your head
before clicking. 724 – 299 = ?
Step 1: Which number will you change
and what tidy number will you change it
into?
724 - 299 = ….
+1
+1
……
725 - ……
300 = 106
….
Step 2: You have changed 299 into a
tidy number by adding 1. What do you
need to do to the other number?
Step 3: What is the answer?
Now try these questions on your own.
Remember to first change the question then answer.
1) 237 – 198 = ?
239 - 200 = 39
2) 435 – 297 = ?
438 + 300 = 138
3) 684 – 499 = ?
685 + 500 = 185
4) 514 – 398 = ?
516 + 400 = 116
5) 317 – 97 = ?
320 + 100 = 220
Click again when you have
completed each question and
the answer will be displayed.
Need some more practise? Try Pg52 of Teacher Tools Addition and
Subtraction Book Numeracy resources