Use Direct Proportion for Calculations

In this activity, we will be looking at direct proportion when it is presented in a table.

 

We have seen this before when we are creating coordinates to draw graphs.

For example y = 2x

 

 

We follow the rule of

y = 2 multiplied by the x value:

1 x 2 = 2

2 x 2 = 4

3 x 2 = 6

4 x 2 = 8

 

 

The graph can now be plotted!

 

Does that make sense?

 

 

Great! 

Now, how about if we have to find the rule?!

Confused? Let's have a look at an example to help!

 

Example

y is directly proportional to x

Complete the table below:

 

 

Answer

We need to look at the table to see where we have the x and the y values.

We can use these to find the rule.

 

We have x = 5 when y = 20

To find the multiplier, we can do 20 ÷ 5 = 4

So, we can see that y = 4x

or

the y values are 4 times bigger than the x values.

 

We can now fill in the gaps:

When x = 3, then y = 4 x 3 = 12

When x = 8, then y = 4 x 8 = 32

 

 

Let's try some more of these!