Step by step
The question
Question: A survey asked boys and girls: yes or no?
Boys: 16 yes, 4 no.
Girls: 12 yes, 8 no.
One person is picked at random.
Find P(yes).
1 Make the table
One row for each group, one column for each answer, and a total for each.
The four counts go inside.
Row totals: 16 plus 4 is 20.
12 plus 8 is 20.
Column totals: 28 and 12.
And 40 people in all.
The totals sit at the edges, in the margins.
2 A total over all
All the yes: 28.
Everyone: 40.
So P of yes is 28/40.
Careful!
The bottom is everyone, 40.
Not only the other 12.
Both 28 and 40 can be divided by 4.
That makes it simpler.
So it is 7/10.
3 Check
Let's check.
The four cells add up to 40, everyone.
The table is right.
And the two cells of yes add up to its total: 16 plus 12 is 28.
So the answer is right!
Let's look back.
First: make the table.
Then: a total over all.
Last: check.