Step by step
The question
Question: Leo has 10 litres of a 60% acid solution.
How many litres of a 20% acid solution must he add to make a 40% acid solution?
1 Find what we know
What do we know?
We have 10 litres of a 60% solution.
The mix must be 40% acid.
What do we want?
How many litres of the 20% solution.
2 Name the unknown
We don't know how many litres of the 20% solution.
So we call it x.
The mix has both: x litres and 10 litres.
So it is x + 10 litres.
Careful!
The mix has the 20% solution in it too.
So it is x + 10 litres, not 10 litres.
Let's put it in a table: how much, the acid part as a decimal, and the acid.
20% is 20/100: 0.2.
60% is 0.6.
Acid = how much times the part.
x times 0.2 is 0.2x.
10 times 0.6 is 6.
The mix: x + 10 litres at 0.4.
Its acid is 0.4(x + 10): all of x + 10 is times 0.4.
3 Write the equation
In words: the acid from the two solutions makes the acid in the mix.
So the equation is 0.2x + 6 = 0.4(x + 10).
Now we solve it.
4 Remove the decimals
Decimals are hard to work with.
Let's get rid of them.
We multiply every part by 10.
We do it on both sides, so they stay equal.
Times 10 moves the point 1 place to the right.
10 × 0.2x = 2x.
10 × 6 = 60.
10 × 0.4(x + 10) = 4(x + 10).
Write the answers.
No more decimals!
5 Open the brackets
The 4 in front of the bracket multiplies every part inside.
So we write 4 times each part.
Work out each multiplication.
6 Get x on one side
There is x on both sides.
We want x on one side only.
So we remove the 2x from the left side.
To remove 2x, we take away 2x.
We do it on both sides, so they stay equal.
2x and − 2x make zero.
Cross them out.
4x − 2x = 2x.
Now we have 60 = 2x + 40.
Let's swap the two sides, so x is on the left.
We want x by itself.
Two things are stuck to x: times 2 and + 40.
The + 40 is on the outside, so it comes off first.
Then the times 2.
7 Remove the + 40
To remove + 40, we take away 40.
We do it on both sides, so they stay equal.
+ 40 and − 40 make zero.
Cross them out.
60 − 40 = 20.
Now we have 2x = 20.
8 Remove the 2
2x means 2 times x.
We want just x.
To remove times 2, we divide by 2.
We do it on both sides, so they stay equal.
2 divided by 2 is 1.
So just x is left.
20 divided by 2 is 10.
So x = 10!
9 Answer the question
So we need 10 litres of the 20% solution.
10 Check
Let's check in the story.
The acid: 2 plus 6 is 8 litres.
40% of the 20 litres of mix is 8 too, like the story says.
Does it make sense?
The mix is between the two solutions.
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.