Posted by Dre on Thursday, October 21, 2010 at 11:11pm.
Suppose only 75% of all drivers in a certain state regularly wear aseat belt. A random sample of 500 drivers is selected. What is theprobability that
a). Between 360 and 400 (inclusive) of the drivers in the sampleregularly wear a seat belt?
b). Fewer than 400 of those in the sample regularly wear a seatbelt?
Part b)is what I am having a hard time with. This is what I did for part a).
using a Normal approximation:
z = (400.5500*0.75) /
sqrt(500*0.75*0.25) = 2.63
z = (359.5500*0.75) /
sqrt(500*0.75*0.25) = 1.60
these numbers correspond to areas of 0.9957 and 0.0548, so the probability is: 0.99570.0548 = 0.9409
I need help solving part b).

statistics  Anonymous, Wednesday, November 3, 2010 at 2:22am
Use the same method you did for part a. Only now you are dealing with just an exclusive upperbound. Just make sure you use the right correction factor and you should be set.