Math Help - Hard Probability

1. Hard Probability

Hi, this is my question:

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

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?

2. Yep.

Although these are all probabilities, you cant be CERTAIN that exactly 90 will be delivered in that time. its jus an average.

3. Originally Posted by andyboy179
Hi, this is my question:

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

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.

4. 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?

then, 0.7 x 0.3=0.21
0.7 x 0.3=0.21
0.21 + 0.21=0.42??

5. your getting the hang of it now!

6. Originally Posted by SpringFan25
your getting the hang of it now!
YES!!! cheers mate!

7. Originally Posted by andyboy179
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?

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