Don’t Subtract Add
I am learning that problems like 34 + ? = 51 and 51 – 34 = ? have
the same answer.
e.g. 34 + 17 = 51
and
51 – 34 = 17
Ashleigh has $17 and needs $25 to buy the new book she wants.
How much more money does she need?
First of all let’s answer this as an addition question. 17 + ? = 25
Step 1: Let’s record what she has already.
Step 2: You can see she needs $3 more to get to twenty
$10
$1
$1
$1
$1
$1
$1
$1
$1
$1
$1
$1
$1
$1
$1
$1
Step 3: Now we need to swap these ten $1 coins for
a $10 note.
Step 4: Now we have $20 so we just need to add $5
then we will have $25.
Amount added
$3 + $5
So $17 + $8 = $25
Ashleigh has read $17 and needs $25 to buy the new book she
wants. How much more money does she need?
We can also answer this as a subtraction question. 25 – 17 = ?
Step 1: Record the amount she needs ($25).
Step 2: We can’t take away $7 because we only have $5 so we must
split up a $10 note to get us another ten $1 coins.
$1
Step 3: Now we can take away $7.
$1
Step 4: Now subtract $10. and you are
left with $8. The amount Ashleigh
must save.
$1
$1
$1
$1
$1
$1
$1
$1
$1
$10
$1
$10
$1
So
$25 - $17 = $8
and
$17 + $8 = $25
Try and work this one out as you go.
Andrew had $23 and spent $5. How much money does he have left.
As an addition question
As a subtraction question
5 + ? = 23
23 – 5 = ?
5 + 5 = 10
First I added 5 to get to
a tidy number.
From the tidy number I
10 + 13 = 23 added 13 to get to 23. I
find it easy to add on
from a tidy number.
Now I add 5 + 13
5 + 13 = 18 together to get my
answer.
23 – 3 = 20
20 – 2 = 18
First I took away 3 to
get back to a tidy
number.
I need to subtract 5. I
have already taken away
3 so now I subtract the
remaining 2. I find it
easy to subtract from a
tidy number.
Notice how both methods get the
same answer.
Now try these questions on your own.
Solve these subtraction questions by first changing them into addition
questions.
Click again when you have
27 + 7 = 34
1) 34 – 27 = ?
completed each question and
the answer will be displayed.
2) 56 – 48 = ?
48 + 8 = 56
3) 72 – 69 = ?
69 + 3 = 72
4) 45 – 36 = ?
36 + 9 = 45
5) 33 – 29 = ?
29 + 4 = 33
Need some more practise? Try Pg45 of Teacher Tools Addition and
Subtraction Book Numeracy resources