Step by step
The question
Question: P = 300 − 25d is the number of pages left to read after d days.
What does 25 mean?
We write the equation big, so we can label it.
1 What the letters mean
P is the number of pages left.
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: 25 times 0 is 0.
300 minus 0 is 300.
d = 1: 25 times 1 is 25.
300 minus 25 is 275.
d = 2: 25 times 2 is 50.
300 minus 50 is 250.
Look at the values of P: 300, 275, 250.
Each time d goes up 1, P goes down by 25.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
And 300?
At d = 0, 25 times 0 is 0.
Only 300 is left: the first number in the table.
So 300 is the number of pages left at 0 days: the start.
Where does minus 25 each time come from?
From 25d: 25 times d, taken away from 300.
Each extra day takes away one more 25.
Careful!
There is a minus before 25.
So the number of pages left goes down, not up.
4 Say it in words
So 25 means: the number of pages left goes down 25 pages for each extra day.
5 Check
Let's check with d = 3.
In words: start at 300, then take away 25 three times.
We get 225.
The equation with d = 3: 25 times 3 is 75.
300 minus 75 gives 225 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.