Step by step
The question
Question: B = 80 − 5h is the battery left in a phone, in percent, after h hours.
What does 5 mean?
We write the equation big, so we can label it.
1 What the letters mean
B is the battery left, in percent.
h is the number of hours.
2 Make a small table
To see what each number does, we try a few hours: 0, 1 and 2.
0 is where it starts.
h = 0: 5 times 0 is 0.
80 minus 0 is 80.
h = 1: 5 times 1 is 5.
80 minus 5 is 75.
h = 2: 5 times 2 is 10.
80 minus 10 is 70.
Look at the values of B: 80, 75, 70.
Each time h goes up 1, B goes down by 5.
The same number every time.
3 Find the numbers in the table
Now we write the equation again, and label its numbers.
And 80?
At h = 0, 5 times 0 is 0.
Only 80 is left: the first number in the table.
So 80 is the battery left at 0 hours: the start.
Where does minus 5 each time come from?
From 5h: 5 times h, taken away from 80.
Each extra hour takes away one more 5.
Careful!
There is a minus before 5.
So the battery left goes down, not up.
4 Say it in words
So 5 means: the battery left goes down 5 percent for each extra hour.
5 Check
Let's check with h = 3.
In words: start at 80, then take away 5 three times.
We get 65.
The equation with h = 3: 5 times 3 is 15.
80 minus 15 gives 65 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.