Question
1) Suppose X ~ N130, 25 . Find; a) P(X 140) b) P(X 120) c) P(130 X 135) 2) The random variable X is normally
1) Suppose
X ~ N130, 25
. Find; a)
P(X 140) b)
P(X 120)
c)
P(130 X 135)
2) The random variable X is normally distributed with mean 500 and standard deviation 100.
Find; (i)
P(X 400)
, (ii)
P(X 620)
(iii) the 90th percentile (iv) the lower and upper
quartiles. Use graphs with labels to illustrate your answers.
3) A radar unit is used to measure speeds of cars on a motorway. The speeds are normally
distributed with a mean of 90 km/hr and a standard deviation of 10 km/hr. What is the
probability that a car picked at random is travelling at more than 100 km/hr?
4) For a certain type of computers, the length of time bewteen charges of the battery is
normally distributed with a mean of 50 hours and a standard deviation of 15 hours. John
owns one of these computers and wants to know the probability that the length of time
will be between 50 and 70 hours
5) Entry to a certain University is determined by a national test. The scores on this test are
normally distributed with a mean of 500 and a standard deviation of 100. Tom wants to
be admitted to this university and he knows that he must score better than at least 70% of
the students who took the test. Tom takes the test and scores 585. Will he be admitted to
this university?
6) A large group of students took a test in Physics and the final grades have a mean of 70
and a standard deviation of 10. If we can approximate the distribution of these grades by a
normal distribution, what percent of the student; (a) scored higher than 80? (b) should
pass the test (grades60)? (c) should fail the test (grades
7) A machine produces bolts which are N(4 0.09) where measurements are in cm. Bolts are
measured accurately and any bolt smaller than 3.5 cm or larger than 4.4 cm is rejected.
Out of 500 bolts how many would be accepted? Ans 430
8) Suppose IQ ~ N(100,22.5).a woman wants to form an Egghead society which only
admits people with the top 1% IQ score. What should she have to set the cut-off in the
test to allow this to happen? Ans 134.9
9) A manufacturer does not know the mean and standard deviation of ball bearing he is
producing. However a sieving system rejects all the bearings larger than 2.4 cm and those
under 1.8 cm in diameter. Out of 1,000 ball bearings, 8% are rejected as too small and
5.5% as too big. What is the mean and standard deviation of the ball bearings produced?
Ans mean=2.08 sigma=0.2
Over a very long period of time, it has been noted that on Friday's 25% of the customers
at the drive-in window at the bank make deposits. What is the probability that it takes 4
customers at the drive-in window before the first one makes a deposit.
2. It is estimated that 45% of people in Fast-Food restaurants order a diet drink with their
lunch. Find the probability that the fourth person orders a diet drink. Also find the
probability that the first diet drinker of th e day occurs before the 5th person.
3. What is the probability of rolling a sum of seven in fewer than three rolls of a pair of
dice? Hint (The random variable, X, is the number of rolls before a sum of 7.)
4. In New York City at rush hour, the chance that a taxicab passes someone and is
available is 15%. a) How many cabs can you expect to pass you for you to find one that
is free and b) what is the probability that more than 10 cabs pass you before you find
one that is free.
5. An urn contains N white and M black balls. Balls are randomly selected, one at a time,
until a black ball is obtained. If we assume that each selected ball is replaced before the
next one is drawn, what is;
a) the probability that exactly n draws are needed?
b) the probability that at least k draws are needed?
c) the expected value and Variance of the number of balls drawn?
6. In a gambling game a player tosses a coin until a head appears. He then receives $2n ,
where n is the number of tosses.
a) What is the probability that the player receives $8.00 in one play of the game?
b) If the player must pay $5.00 to play, what is the win/loss per game?
7. An oil prospector will drill a succession of holes in a given area to find a productive
well. The probability of success is 0.2.
a) What is the probability that the 3rd hole drilled is the first to yield a productive well?
b) If the prospector can afford to drill at most 10 well, what is the probability that he will
fail to find a productive well?
8. A well-travelled highway has itstraffic lights green for 82% of the time. If a person
travelling the road goes through 8 traffic intersections, complete the chart to find a) the
probability that the first red light occur on the nth traffic light and b) the cumulative
probability that the person will hit the red light on or before the nth traffic light.
9. An oil prospector will drill a succession of holes in a given area to find a productive
well. The probability of success is 0.2.
a) What is the probability that the 3rd hole drilled is the first to yield a productive well?
b) If the prospector can afford to drill at most 10 well, what is the probability that he will
fail to find a productive well?
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