00:01
Okay, so for part a, we need exactly 18 refusals.
00:04
Okay, so that's going to be 20, choose 18.
00:08
And the probability of refusal is 80%, so that's 0 .80 to the 18th power, times 0 .2 squared.
00:23
Okay, so first i need 20, choose 18.
00:32
That's going to be 190.
00:37
Okay, so plugging that in to get our exactly 18, just put all that in the calculator.
00:42
So it's 190 times 0 .8 to the 18th power, time 0 .2.
00:55
The probability of getting exactly 18 refusals is 13 .69 % of the time.
01:09
Okay.
01:09
For b, exactly 19 refuse.
01:15
So it's going to be 20 choose 19, which is just 20 times 0 .8 to the 19th power.
01:29
Times 0 .2...