Download Multiple ways to add and subtract

Multiple ways to add and subtract
I am learning to use multiplication and division to solve addition
and subtraction problems.
Below is an array. You can see it has 4 rows and 6 faces in each
row. We can refer to this as 4 X 6 = 24
Here is an array showing 2 X 6 = 12
Let’s add the two arrays together.
24 + 12 = 36
4 X 6 = 24 plus 2 X 6 = 12.
Watch what happens when we
combine the two arrays.
Now we have 6 X 6 = 36
Let’s look a little more closely.
(4 X 6) + (2 X 6) = 6 x 6
We can combine the 4 and the 2 together
because they both have the common
multiple of 6.
Now let’s use this strategy for an actual question. Craig has 28
marbles, Sarah has 9 marbles and Joe has 45 marbles. How many
marbles do they have in total?
27 + 9 + 45 = ?
Step 1: Think of a number which is a factor of
all these numbers. 9
(3 X 9) + (1 X 9) + (5 X 9) Step 2: We can combine these to make the
question below.
9 X 9 = 81
We can also use this strategy for subtraction. Alicia has read 56 books
this year. 24 of these were fiction books. How many of these were not
fiction.
56 – 24 = ?
Step 1: Think of a number which is a factor of both
56 and 24. 8
(7 X 8) - (3 X 8) = ?
Step 2: (7 groups of 8) – (3 groups of 8) = (4 groups of 8)
4 X 8 = 32
Now try these questions on your own. Remember to first find a common
factor.
1) 66 + 42 + 12 = ?
(11 X 6) + (7 X 6) + (2 X 6) = 20 X 6
120
2) 25 + 35 + 50 = ?
(5 X 5) + (7 X 5) + (10 X 5) = 22 X 5
110
3) 16 + 28 + 12 = ?
(4 X 4) + (7 X 4) + (3 X 4) = 14 X 4
56
4) 81 - 63 = ?
(9 X 9) - (7 X 9) = 2 X 9
18
Need some more practise? Try Pg68 of Teacher Tools Addition and
Subtraction Book Numeracy resources