00:01
Okay, we have here seven questions and you're guessing each one and there are five choices per question.
00:07
So it's a binomial distribution question and p of being correct is one chance out of five or 0 .2.
00:19
So p of wrong then 0 .8 or 4 out of 5.
00:26
Now, i want to work out p of at least 5 correct, which means p of 5, 6 or 7 correct.
00:43
You could work out each one separately and add them up, but it's less work.
00:48
You can do the following.
00:49
Go backwards.
00:51
So at least 5 correct is certain, so 1, unless, so minus, you get 0, 1, 2, 2, 3 or 4.
01:04
So five or more then correct.
01:07
Same thing as saying 1 minus 4 or less correct.
01:13
Now, this one here i can work out in one step, well not one step, but on a calculator.
01:22
And it's quite straightforward.
01:24
On the ti 84 plus, what you will use is the function binom cdf.
01:34
That works out each one separately and adds them up for you.
01:37
So binom cdf and that you can find by pressing second on the ti84 plus followed by vars vars on the right just below the arrow keys.
01:52
That brings up the menu for distributions...