Name: _______Answers____________________
Geometric Distributions WS
AP Statistics
1) Only 4% of people have Type AB blood.
a) On average, how many donors must be checked to first find someone with Type AB blood?
1/.04 = 25
b) What is the probability that at most five people will have to be checked before first finding someone with Type AB
blood?
P(X < 5) - .1846
c) What is the probability that we won’t find a Type AB donor before the tenth person?
P(X > 10) = .6925
d) What is the probability that out of 10 donors at least one have Type AB blood?
1 – (.96)10 = .3352
2) Cards with athlete’s pictures are prizes in a certain type of cereal. There is a 20% chance that the card has a picture of
Tiger Woods on it.
a) What is the probability that the first box that has Tiger’s picture is the third box that you open?
P (X = 3) = .128
b) What is the probability that the first box that has Tiger’s picture is at most the third box that you open?
P(X < 3) = .488
c) How many boxes of cereal can you expect to open before you first find a picture of Tiger?
µ = 1/.2 = 5
d) What is the standard deviation of this distribution?
σ = 4.47
3) Police estimate that 80% of drivers now wear their seatbelts. The set up a safety roadblock, stopping cars to check for
seatbelt use.
a) How many cars do they expect to stop before finding a driver whose seatbelt is not buckled?
µ = 1/.2 = 5
b) What is the probability that the first unbelted driver is in the 6th car stopped?
P (X = 6) = .0655
c) What is the probability that the first 10 drivers are all wearing their seatbelts?
P(all) = .810 = .1074
d) If they stop 30 cars during the first hour, find the mean and standard deviation of the number of drivers expected
to be wearing seatbelts.
(binomial) µ = 30(.8) = 24
σ=
np(1  p)  30*.8*.2  2.19
4) Suppose a computer chip manufacturer rejects 2% of chips produced because they fail pre-sale testing.
a) What is the probability that the fifth chip you test is the first bad one you find?
P(x = 5) = .0184
b) What is the probability that the first bad chip you find is at most the tenth chip you test?
P(x < 10) = .1829
c) Find the number of chips that you expect to test before finding the first bad chip.
µ = 1/.02 = 50