Step by step
The question
Question: In a sample of 120 phones, 9 were faulty.
About how many of 2,400 are faulty?
1 The fraction in the sample
9 out of 120: that is 9/120.
9120
The 120 phones are a sample: a small group that stands for everyone.
sample: 120 phones
9 and 120 can both be divided by 3.
So it is 3/40.
= 9 ÷ 3120 ÷ 3 = 340
2 Use it for everyone
We expect the same fraction of all 2,400: 3/40 of 2,400.
Of means times.
= 340 × 2,400
2,400 divided by 40 is 60.
Then times 3.
= 3 × 60
2,400 ÷ 40 = 60
That is about 180.
= ≈ 180
Careful!
A sample only shows about how many.
So the answer is a good guess, not an exact count.
Careful: about, not exactly
So about 180 phones are faulty.
3 Check
Let's check: 180 out of 2,400.
Divide the top and the bottom by 60: 3/40, the same fraction as the sample.
So the answer is right!
1802,400 = 180 ÷ 602,400 ÷ 60 = 340
Let's look back.
First: the fraction in the sample.
Then: use it for everyone.
Last: check.
Answer
Careful: about, not exactly