A plumber costs $40 plus $25 per hour. Write the equation.

Step by step

The question

Question: A plumber costs $40 plus $25 per hour.

Write the equation.

1 Which depends on which

The cost depends on the number of hours.

So x is the hours: the input.

y is the cost: the output.

2 Rate of change

Rate of change means: how much y changes when x goes up by 1.

Each hour, it goes up by 25.

So m is 25.

3 Start value

The start value is y when x is 0.

At the start, with 0 hours, it is 40.

Careful!

The 25 is per hour: it goes with x.

The 40 is only once.

4 Write the equation

A line is y = m x plus b: m is the rate of change, b is the start value.

So the equation is y = 25x + 40.

5 Check

Let's check with 4 hours: the equation gives 140.

In the story: 40, plus 25, plus 25, plus 25, plus 25: 140.

So the answer is right!

Let's look back.

First: which depends on which.

Then: rate of change.

Then: start value.

Then: write the equation.

Last: check.