# Binomial Propability

• Sep 14th 2010, 07:37 PM
Lukybear
Binomial Propability
During winter it rains on average 18 out of 30 days. Five winter days are selected at random. Find correct probability that:

a) First two days choosen will be fine and the remainder wet.

I attempted 5C2 (12/30)^2 (18/30)^3 counting exactly 2 days fine and 3 days rain.

Then i calculated probability first 2 days raining, which is (2!/2!3!)/5!/(2!3!) = (2!x3!x2!)/5!

to attain 0.06912 which is not same as answer. Where did i go wrong?
• Sep 14th 2010, 07:44 PM
mr fantastic
Quote:

Originally Posted by Lukybear
During winter it rains on average 18 out of 30 days. Five winter days are selected at random. Find correct probability that:

a) First two days choosen will be fine and the remainder wet.

I attempted 5C2 (12/30)^2 (18/30)^3 counting exactly 2 days fine and 3 days rain.

Then i calculated probability first 2 days raining, which is (2!/2!3!)/5!/(2!3!) = (2!x3!x2!)/5!

to attain 0.06912 which is not same as answer. Where did i go wrong?

The order matters:

(18/30)^2 (1 - 18/30)^3