Step by step
The question
Question: T = 72 − 3m is the temperature of the tea, in degrees, after m minutes.
What does 72 mean?
We write the equation big, so we can label it.
1 What the letters mean
T is the temperature of the tea, in degrees.
m 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.
m = 0: 3 times 0 is 0.
72 minus 0 is 72.
m = 1: 3 times 1 is 3.
72 minus 3 is 69.
m = 2: 3 times 2 is 6.
72 minus 6 is 66.
Look at the values of T: 72, 69, 66.
Each time m goes up 1, T goes down by 3.
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 3 each time come from?
From 3m: 3 times m, taken away from 72.
Each extra minute takes away one more 3.
And 72?
At m = 0, 3 times 0 is 0.
Only 72 is left: the first number in the table.
So 72 is the temperature of the tea at 0 minutes: the start.
Careful!
72 is not multiplied by m.
It is there once, even at 0 minutes.
4 Say it in words
So 72 means: the temperature of the tea is 72 degrees at 0 minutes.
That is the start.
5 Check
Let's check with m = 3.
In words: start at 72, then take away 3 three times.
We get 63.
The equation with m = 3: 3 times 3 is 9.
72 minus 9 gives 63 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.