Step by step
The question
Question: C = 0.25m + 40 is the cost, in dollars, of a phone plan with m minutes of calls.
What does 0.25 mean?
We write the equation big, so we can label it.
1 What the letters mean
C is the cost of a phone plan, in dollars.
m is the number of minutes.
2 Make a small table
To see what each number does, we try a few minutes: 0, 1 and 2.
0 is where it starts.
m = 0: 0.25 times 0 is 0.
Plus 40 is 40.
m = 1: 0.25 times 1 is 0.25.
Plus 40 is 40.25.
m = 2: 0.25 times 2 is 0.5.
Plus 40 is 40.5.
Look at the values of C: 40, 40.25, 40.5.
Each time m goes up 1, C goes up by 0.25.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
And 40?
At m = 0, 0.25 times 0 is 0.
Only 40 is left: the first number in the table.
So 40 is the cost of a phone plan at 0 minutes: the start.
Where does plus 0.25 each time come from?
From 0.25m: 0.25 times m.
Each extra minute adds one more 0.25.
Careful!
0.25 is multiplied by m, so each minute adds another 0.25.
The start is the number alone.
4 Say it in words
So 0.25 means: the cost of a phone plan goes up 0.25 dollars for each extra minute.
5 Check
Let's check with m = 3.
In words: start at 40, then add 0.25 three times.
We get 40.75.
The equation with m = 3: 0.25 times 3 is 0.75.
Plus 40 gives 40.75 too.
So the meaning is right!
Let's look back.
First: what the letters mean.
Then: make a small table.
Then: find the numbers in the table.
Then: say it in words.
Last: check.