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To read the table for data (initially) you simply read along the rows or the columns to find out the answer.
To then use that for probability, what you need to do is take your answer (found using the above method) and work it out as a percentage of all the answers in that row or column.
So its hard to draw a table here but imagine that this is a table listing boys and girls in year 10 and in year 11. (So the rows are boys and girls, the columns are Y10 and Y11. The numbers in the table are the no of boys and girls in each year
Boys - 100 in Y10 ; 150 in Y11
Girls - 80 in Y10 ; 120 in Y11
QUESTION - If you picked a girls at random, what is the probability that she is in Y 10?
There are 80 girls in Y10 (by reading along the girls row in the table).
And there are a total of 80 + 120 girls in total in Y 10 and 11 = 200 girls
So the chance you get a Y 10 girl is 80 / 200 = 2 / 5 = 0.4 or 40%