Step by step
The question
Question: G = 16 − 2d is the gas left in a car, in gallons, after d days.
What does 2 mean?
We write the equation big, so we can label it.
1 What the letters mean
G is the gas left, in gallons.
d is the number of days.
2 Make a small table
To see what each number does, we try a few days: 0, 1 and 2.
0 is where it starts.
d = 0: 2 times 0 is 0.
16 minus 0 is 16.
d = 1: 2 times 1 is 2.
16 minus 2 is 14.
d = 2: 2 times 2 is 4.
16 minus 4 is 12.
Look at the values of G: 16, 14, 12.
Each time d goes up 1, G goes down by 2.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
And 16?
At d = 0, 2 times 0 is 0.
Only 16 is left: the first number in the table.
So 16 is the gas left at 0 days: the start.
Where does minus 2 each time come from?
From 2d: 2 times d, taken away from 16.
Each extra day takes away one more 2.
Careful!
There is a minus before 2.
So the gas left goes down, not up.
4 Say it in words
So 2 means: the gas left goes down 2 gallons for each extra day.
5 Check
Let's check with d = 3.
In words: start at 16, then take away 2 three times.
We get 10.
The equation with d = 3: 2 times 3 is 6.
16 minus 6 gives 10 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.