3 pizzas cost $36 and 5 pizzas cost $55. Is the cost proportional to…

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The question

Question: 3 pizzas cost $36, and 5 cost $55: is the cost proportional to the number of pizzas?

Let x be the number of pizzas.

Let y be the cost.

Proportional means: y divided by x is always the same number.

We call that number k.

Then y = k times x.

1 Divide y by x

For each pair we work out y divided by x.

36/3 is 12.

55/5 is 11.

2 Compare the answers

The answers are not the same.

So it is not proportional.

Careful!

y goes up by the same amount each time.

But that is not enough.

y divided by x must stay the same.

3 Check

Let's check: the first two pairs already give different answers.

So the answer is right!

Let's look back.

First: divide y by x.

Then: compare the answers.

Last: check.