Question 184191
The table shows the results of a survey that asked students from three grade levels to pick their favorite fruit from those listed. Find P(Banana\11). 
......10th 11th 12th
apple. 23.. 35.. 21
orange 16.. 12.. 30
pear... 8... 7.. 12
banana.32...24.. 18 
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P(banana | 11th grade) = 24/78 = 0.3077
Note: When you have a contingency table like this, if 11th is
the conditional, the total of the 11th column is the denominator
of the probability.
You want the Probability of banana in that 11th column.  it
is 24/78
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Another way to do the problem:
P(banana | 11th) = P(banana and 11th)/P(11th) = (24/grand total)/(78/grand total)
The grand totals cancel and you end up with 24/78
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Cheers,
Stan H.