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Math Help - Hard Probability

  1. #1
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    Hard Probability

    Hi, this is my question:

    The table shows the probability for the delivery time of letters posted first class.

    Hard Probability-z.jpg

    A) 100 letters are posted first class.
    How many will be delivered in 1 or 2 days?

    would i do
    0.7 x 100=70
    0.2 x 100=20
    70 + 20=90?
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  2. #2
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    Yep.


    Although these are all probabilities, you cant be CERTAIN that exactly 90 will be delivered in that time. its jus an average.
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  3. #3
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    Quote Originally Posted by andyboy179 View Post
    Hi, this is my question:

    The table shows the probability for the delivery time of letters posted first class.

    Click image for larger version. 

Name:	z.jpg 
Views:	8 
Size:	27.5 KB 
ID:	17069

    A) 100 letters are posted first class.
    How many will be delivered in 1 or 2 days?

    would i do
    0.7 x 100=70
    0.2 x 100=20
    70 + 20=90?
    Or....

    0.1(100)=10 on average will be delivered in 3 or more days,

    hence 100-10=90 on average will be delivered in less than 3 days.
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  4. #4
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    thanks, here is the hard part:
    Two letters are posted first class in different towns.
    Whats is the probability that only one will be delivered in one day?

    would this work?

    Hard Probability-zz.jpg

    then, 0.7 x 0.3=0.21
    0.7 x 0.3=0.21
    0.21 + 0.21=0.42??
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  5. #5
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    your getting the hang of it now!
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  6. #6
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    Quote Originally Posted by SpringFan25 View Post
    your getting the hang of it now!
    YES!!! cheers mate!
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  7. #7
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    Quote Originally Posted by andyboy179 View Post
    thanks, here is the hard part:
    Two letters are posted first class in different towns.
    Whats is the probability that only one will be delivered in one day?

    would this work?

    Click image for larger version. 

Name:	zz.jpg 
Views:	6 
Size:	31.1 KB 
ID:	17070

    then, 0.7 x 0.3=0.21
    0.7 x 0.3=0.21
    0.21 + 0.21=0.42??
    Only one letter means it could be either one.
    That's the idea

    Notice that all probabilities sum to 1.

    Both letters arrive in 1 day is (0.7)(0.7)=0.49
    Neither arrive in a day is (0.3)(0.3)=0.09

    There are no other options.

    0.42+0.49+0.9=1.0
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