Hello, r7iris!

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: .

Multiply by

Therefore, there must be9sophomore girls.