Step by step
The question
Question: C = 12h + 30 is the cost, in dollars, to rent a bike for h hours.
What does 30 mean?
We write the equation big, so we can label it.
1 What the letters mean
C is the cost, in dollars.
h is the number of hours.
2 Make a small table
To see what each number does, we try a few hours: 0, 1 and 2.
0 is where it starts.
h = 0: 12 times 0 is 0.
Plus 30 is 30.
h = 1: 12 times 1 is 12.
Plus 30 is 42.
h = 2: 12 times 2 is 24.
Plus 30 is 54.
Look at the values of C: 30, 42, 54.
Each time h goes up 1, C goes up by 12.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
Where does plus 12 each time come from?
From 12h: 12 times h.
Each extra hour adds one more 12.
And 30?
At h = 0, 12 times 0 is 0.
Only 30 is left: the first number in the table.
So 30 is the cost at 0 hours: the start.
Careful!
30 is not multiplied by h.
It is there once, even at 0 hours.
4 Say it in words
So 30 means: the cost is 30 dollars at 0 hours.
That is the start.
5 Check
Let's check with h = 3.
In words: start at 30, then add 12 three times.
We get 66.
The equation with h = 3: 12 times 3 is 36.
Plus 30 gives 66 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.