Step by step
The question
Question: W = 500 − 20t is the water left in a tank, in liters, after t minutes.
What does 20 mean?
We write the equation big, so we can label it.
1 What the letters mean
W is the water left, in liters.
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: 20 times 0 is 0.
500 minus 0 is 500.
t = 1: 20 times 1 is 20.
500 minus 20 is 480.
t = 2: 20 times 2 is 40.
500 minus 40 is 460.
Look at the values of W: 500, 480, 460.
Each time t goes up 1, W goes down by 20.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
And 500?
At t = 0, 20 times 0 is 0.
Only 500 is left: the first number in the table.
So 500 is the water left at 0 minutes: the start.
Where does minus 20 each time come from?
From 20t: 20 times t, taken away from 500.
Each extra minute takes away one more 20.
Careful!
There is a minus before 20.
So the water left goes down, not up.
4 Say it in words
So 20 means: the water left goes down 20 liters for each extra minute.
5 Check
Let's check with t = 3.
In words: start at 500, then take away 20 three times.
We get 440.
The equation with t = 3: 20 times 3 is 60.
500 minus 60 gives 440 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.