Two events, and , are independent if: .In a class there are 4 freshman boys, 6 freshman girls and 6 sophomore boys.
How many sophomore girls must be present if sex and class are to be independent
when a student is selected at random?
Let = number of sophomore girls.
Tabulate the information:
We have: .
If and are independent,
. . then: .
So we have: .
Therefore, there must be 9 sophomore girls.