# I need some help

1. Nov 5, 2006

### tunganhtr

Hi, I am a 2nd year student in Economics. I have some difficulties with probability. Here is a problem that I cannot solve and I need some help.

It is supposed that only 50% of the students of a sample will go up to the level master. One chooses by chance 18 students.
Which is the probability so that at least 10 go up to the level master.

I want just to know in this exercise, which is the law of probability that one must apply.
Thank you for your assistance, and I'm so sorry for my bad English.

2. Nov 6, 2006

### marcmtlca

$$\Large \sum_{x=10}^{18}(.5)^x(.5)^{18-x}$$

3. Nov 7, 2006

### tunganhtr

Thanks you very much for the answer. It helps me a lot !
:)

4. Nov 8, 2006

### HallsofIvy

Staff Emeritus
tunganhtr, I feel I really should point out to you that marcmtlca's answer is INCORRECT. It doesn't help you as much as you think.

(In fact,
$$\Large \sum_{x=10}^{18}(.5)^x(.5)^{18-x}$$
$$= \Large \sum_{x= 10}^{18}(.5)^{18}$$
$$= 9(.5)^{18}= 0.00004196$$
which is much too small.)

Do you remember why this is called a "binomial" distribution?