Step by step
The question
Question: Eva can weed a garden in 5 hours.
Tom can weed it in 20 hours.
How many hours will it take them to weed it together?
1 Find what we know
What do we know?
Eva does the whole job alone in 5 hours.
Tom does it alone in 20 hours.
What do we want?
How long it takes them together.
2 Name the unknown
We don't know how long it takes together.
So we call it x hours.
Careful!
Many people add the times: 25 hours.
But two working together are faster than one.
So together it takes less than 5 hours.
The whole garden is 1 whole job.
We split it into parts.
Let's put it in a table: the time alone, and the part done.
Eva does the whole job in 5 hours.
So in 1 hour, Eva does 1 part out of 5.
That is 1/5.
Tom does 1 part out of 20 in 1 hour: 1/20.
In x hours, Eva does x times as much: x/5.
And Tom does x/20.
3 Write the equation
In words: Eva's part plus Tom's part makes the whole job.
So the equation is x/5 + x/20 = 1.
Now we solve it.
4 Remove the fractions
The bottom numbers are 5 and 20.
20 can be divided by all of them.
We multiply every part by 20.
We do it on both sides, so they stay equal.
20 divided by 5 is 4.
20 divided by 20 is 1.
Write what is left.
Do the multiplications.
No more fractions!
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.
4x + x = 5x.
Now we have 5x = 20.
6 Remove the 5
5x means 5 times x.
We want just x.
To remove times 5, we divide by 5.
We do it on both sides, so they stay equal.
5 divided by 5 is 1.
So just x is left.
20 divided by 5 is 4.
So x = 4!
7 Answer the question
So together it takes 4 hours.
8 Check
Let's check in the story.
In 4 hours, Eva does 4/5.
That is 4/5 of the garden.
In 4 hours, Tom does 4/20.
That is 1/5 of the garden.
Together: 4/5 plus 1/5 is 5/5.
That is 1 whole garden, like the story says.
Does it make sense?
Together is faster than Eva alone, who needs 5 hours.
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.