Step by step
The question
Question: Tea costs $8 per kg and coffee costs $20 per kg.
A shop mixes them to make 6 kg of blend that costs $12 per kg.
How many kg of each does it use?
1 Find what we know
What do we know?
Tea: $8 per kg.
Coffee: $20 per kg.
They make 6 kg of blend, at $12 per kg.
What do we want?
How many kg of coffee, and how many of tea.
2 Name the unknown
We don't know how many kg of coffee.
So we call it x.
The rest is tea.
6 kg in all, take away the x kg of coffee: 6 − x.
Let's put it in a table: how much, the price, and the cost.
Coffee: x kg at $20.
Tea: 6 − x kg at $8.
Cost = how much times the price.
20 times x is 20x.
8 times all of 6 − x is 8(6 − x).
We keep 6 − x together in brackets.
The mix: 6 kg at $12.
6 times 12 is $72.
3 Write the equation
In words: the coffee cost plus the tea cost makes the cost of the mix.
So the equation is 20x + 8(6 − x) = 72.
Now we solve it.
Careful!
Many people take half of each.
But half and half costs (8 + 20) ÷ 2 = $14 per kg, not $12.
4 Open the brackets
The 8 in front of the bracket multiplies every part inside.
So we write 8 times each part.
Work out each multiplication.
5 Group like terms
Terms are the parts between the + and − signs.
Like terms are the same kind: same letters, same powers.
All the x terms go together.
20x − 8x = 12x.
Now we have 12x + 48 = 72.
We want x by itself.
Two things are stuck to x: times 12 and + 48.
The + 48 is on the outside, so it comes off first.
Then the times 12.
6 Remove the + 48
To remove + 48, we take away 48.
We do it on both sides, so they stay equal.
+ 48 and − 48 make zero.
Cross them out.
72 − 48 = 24.
Now we have 12x = 24.
7 Remove the 12
12x means 12 times x.
We want just x.
To remove times 12, we divide by 12.
We do it on both sides, so they stay equal.
12 divided by 12 is 1.
So just x is left.
24 divided by 12 is 2.
So x = 2!
8 Answer the question
Tea: 6 − 2 = 4.
So 2 kg of coffee and 4 kg of tea.
9 Check
Let's check in the story.
2 kg and 4 kg make 6 kg, like the story says.
The cost: $40 plus $32 is $72.
That is 6 kg at $12, like the story says.
Does it make sense?
The mix costs between the two prices, $8 and $20.
That makes sense.
So the answer is right!
Let's look back.
First: find what we know.
Then: name the unknown.
Then: write the equation.
Then: solve it.
Then: answer the question.
Last: check.