Step by step
The question
Question: H = 120 − 1.5t is the height of a candle, in millimeters, after t minutes.
What does 1.5 mean?
We write the equation big, so we can label it.
1 What the letters mean
H is the height of a candle, in millimeters.
t 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.
t = 0: 1.5 times 0 is 0.
120 minus 0 is 120.
t = 1: 1.5 times 1 is 1.5.
120 minus 1.5 is 118.5.
t = 2: 1.5 times 2 is 3.
120 minus 3 is 117.
Look at the values of H: 120, 118.5, 117.
Each time t goes up 1, H goes down by 1.5.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
And 120?
At t = 0, 1.5 times 0 is 0.
Only 120 is left: the first number in the table.
So 120 is the height of a candle at 0 minutes: the start.
Where does minus 1.5 each time come from?
From 1.5t: 1.5 times t, taken away from 120.
Each extra minute takes away one more 1.5.
Careful!
There is a minus before 1.5.
So the height of a candle goes down, not up.
4 Say it in words
So 1.5 means: the height of a candle goes down 1.5 millimeters for each extra minute.
5 Check
Let's check with t = 3.
In words: start at 120, then take away 1.5 three times.
We get 115.5.
The equation with t = 3: 1.5 times 3 is 4.5.
120 minus 4.5 gives 115.5 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.