There is an example in OG Edition 13 Page 120 for Discrete Probability that goes as...
Consider an experiment with events A, B, and C for which P(A) = 0.23, P(B) = 0.40, and P(C) = 0.85. Also, suppose that events A and B are mutually exclusive and events B and C are independent. Then
Step 1: P(A or B) = P(A)+P(B) (Since A and B are mutually exclusive)
= 0.23+0.40
= 0.63
Step 2: P(B or C) = P(B)+P(C)-P(B)P(C) (by independence)
= 0.40+0.85-(0.40)(0.85)
= 0.91
Step 3: P(A or C) and P(A and C) cannot be determined using the information given. But it can be determined that A and C are "not mutually exclusive" since P(A)+P(C)=1.08, which is greater than 1, and therefore cannot equal P(A or C); from this it follows that P(A and C) >= 0.08. Once can also deduce that P(A and C) <= P(A) = 0.23, since AC is a subset of A, and that P(A or C) >= P(C) = 0.85 since C is a subset of AC. Thus, one can conclude that 0.85 <= P(A or C) <= 1 and 0.08 <= P(A and C) <= 0.23
Step 1 and Step 2 are obviously clear...
Can someone explain step 3? It doesn't make any sense to me at all...
"it can be determined that A and C are "not mutually exclusive" since P(A)+P(C)=1.08" - All this means is that P(A) + P(C) > 1 but how can you infer that A and C ARE NOT MUTUALLY EXCLUSIVE based on this fact.(Is this a property/rule for 2 events that are not mutually exclusive?)
Furthermore it says
"which is greater than 1, and therefore cannot equal P(A or C)" - What???How???Why???
then
"from this it follows that P(A and C) >= 0.08" - How?(I tried to put it this way but did not arrive at any meaningful conclusion. - P(A and C)>=1.08-1 which is P(A)+P(C) - 1.
then
"Once can also deduce that P(A and C) <= P(A) = 0.23 , since AC is a subset of A" - How?(and why is it not <=P(C) and how is AC is a subset of A?)
then
and that P(A or C) >= P(C) = 0.85 since C is a subset of AC - How???Why????
Finally,
"Thus, one can conclude that 0.85 <= P(A or C) <= 1 and 0.08 <= P(A and C) <= 0.23 " - How???
There is something NOT right with this example or some subtle fact/property that I am missing to understand.
Any help is greatly appreciated.
Thanks in advance
Consider an experiment with events A, B, and C for which P(A) = 0.23, P(B) = 0.40, and P(C) = 0.85. Also, suppose that events A and B are mutually exclusive and events B and C are independent. Then
Step 1: P(A or B) = P(A)+P(B) (Since A and B are mutually exclusive)
= 0.23+0.40
= 0.63
Step 2: P(B or C) = P(B)+P(C)-P(B)P(C) (by independence)
= 0.40+0.85-(0.40)(0.85)
= 0.91
Step 3: P(A or C) and P(A and C) cannot be determined using the information given. But it can be determined that A and C are "not mutually exclusive" since P(A)+P(C)=1.08, which is greater than 1, and therefore cannot equal P(A or C); from this it follows that P(A and C) >= 0.08. Once can also deduce that P(A and C) <= P(A) = 0.23, since AC is a subset of A, and that P(A or C) >= P(C) = 0.85 since C is a subset of AC. Thus, one can conclude that 0.85 <= P(A or C) <= 1 and 0.08 <= P(A and C) <= 0.23
Step 1 and Step 2 are obviously clear...
Can someone explain step 3? It doesn't make any sense to me at all...
"it can be determined that A and C are "not mutually exclusive" since P(A)+P(C)=1.08" - All this means is that P(A) + P(C) > 1 but how can you infer that A and C ARE NOT MUTUALLY EXCLUSIVE based on this fact.(Is this a property/rule for 2 events that are not mutually exclusive?)
Furthermore it says
"which is greater than 1, and therefore cannot equal P(A or C)" - What???How???Why???
then
"from this it follows that P(A and C) >= 0.08" - How?(I tried to put it this way but did not arrive at any meaningful conclusion. - P(A and C)>=1.08-1 which is P(A)+P(C) - 1.
then
"Once can also deduce that P(A and C) <= P(A) = 0.23 , since AC is a subset of A" - How?(and why is it not <=P(C) and how is AC is a subset of A?)
then
and that P(A or C) >= P(C) = 0.85 since C is a subset of AC - How???Why????
Finally,
"Thus, one can conclude that 0.85 <= P(A or C) <= 1 and 0.08 <= P(A and C) <= 0.23 " - How???
There is something NOT right with this example or some subtle fact/property that I am missing to understand.
Any help is greatly appreciated.
Thanks in advance