Algebra please help!

Question

Bobby and his friends were planning their trip to a water park. The park offers 13 total rides including 4 lazy rivers. If Bobby randomly chooses to go on 4 of the rides, what is the probability that exactly 2 of the chosen rides are lazy rivers?

Answer 1

There are 4 lazy river rides and 13 total, so 9 others. Thus, the probability of picking 2 lazy rivers of 4 rides is calculated using the hypergeometric distribution:

C(4,2)*C(9,2)/C(13,4) = 6*36/715 = 216/715 = .3021

Note that calculators and Excel these days have the hyper-geometric distribution.

For example, in Excel, we can use =HYPGEOM.DIST(2,4,4,13,0)