Introduction to Algebra - Multiplication
Please read Introduction to Algebra first
A Puzzle
What is the missing number?
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|
× | 4 | = | 8 |
The answer is 2, right? Because 2 × 4 = 8.
Well, in Algebra we don't use blank boxes, we use a letter. So we might write:
| x | × | 4 | = | 8 |
But the "x" looks like the "×" ... that can be very confusing ... so in Algebra we don't use the multiply symbol (×) between numbers and letters:
We put the number next to the letter to mean multiply:
| 4x | = | 8 |
In English we say "four x equals eight", meaning that 4 x's make 8.
And the answer is written:
| x | = | 2 |
How to Solve
Instead of saying "obviously x=2", use this neat step-by-step approach:
- Work out what to remove to get "x = ..."
- Remove it by doing the reverse
- Do that to both sides
And what is the reverse of multiplying? Dividing!
Have a look at this example:
to remove
the "4"
To remove it, do
the reverse, in
this case divide by 4
Do it to
both sides
Which is ...
Solved!
Why did we divide by 4 on both sides?
Because of the need for balance ...
 |
| In Balance |
| Divide Left by 4 |
 |
| Out of Balance! |
| Divide Right by 4 Also |
 |
| In Balance Again |
Just remember ...
| To keep the balance, what we do to one side of the "=" we should also do to the other side! |
Another Puzzle
Solve this one:| x | / | 3 | = | 5 |
What we want is an answer like "x = ...", but the divide by 3 is in the way of that!
If we multiply by 3 we can cancel out the divide by 3 (because 3/3=1)
More Complicated Example
How do we solve this?
| x | / | 3 | + | 2 | = | 5 |
It might look hard, but not if we solve it in stages.
First let us get rid of the "+2":
Now, get rid of the "/3":
When you get more experienced:
When you get more experienced, you can solve it like this:
Real World Example
Example: Sam bought 3 boxes of chocolates online.
Postage was $9 and the total cost was $45.
How much was each box?
Let's use x for the price of each box.
3 times x plus $9 is $45:
3x + 9 = 45
Let's solve!
So each box was $12
Advanced: we can also do the "divide by 3" first (but we must do it to all terms):
Same answer!
Have a Try Yourself
Now practice on this Algebra (Two Steps to Solve) Worksheet and then check your answers on the page after. Try to use the steps we have shown you here, rather than just guessing!