Step by step
The question
Question: V = 1200 − 150y is the value, in dollars, of a laptop y years after it is bought.
What does 1200 mean?
We write the equation big, so we can label it.
1 What the letters mean
V is the value of a laptop, in dollars.
y is the number of years.
2 Make a small table
To see what each number does, we try a few years: 0, 1 and 2.
0 is where it starts.
y = 0: 150 times 0 is 0.
1200 minus 0 is 1200.
y = 1: 150 times 1 is 150.
1200 minus 150 is 1050.
y = 2: 150 times 2 is 300.
1200 minus 300 is 900.
Look at the values of V: 1200, 1050, 900.
Each time y goes up 1, V goes down by 150.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
Where does minus 150 each time come from?
From 150y: 150 times y, taken away from 1200.
Each extra year takes away one more 150.
And 1200?
At y = 0, 150 times 0 is 0.
Only 1200 is left: the first number in the table.
So 1200 is the value of a laptop at 0 years: the start.
Careful!
1200 is not multiplied by y.
It is there once, even at 0 years.
4 Say it in words
So 1200 means: the value of a laptop is 1200 dollars at 0 years.
That is the start.
5 Check
Let's check with y = 3.
In words: start at 1200, then take away 150 three times.
We get 750.
The equation with y = 3: 150 times 3 is 450.
1200 minus 450 gives 750 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.