Posted by Jessi on .
Can some one please help me with this 3 problems I have a hard time with probabilities I'm studying for my midterm. Thank you
1) Urn A has balls numbered 1 through 7. Urn B has balls numbered 1 through 5. A ball is selected at random from urn A and then a ball is selected at random from urn B. What is the probability that a 4 is drawn from A followed by a 2 from B?
2)Mark can remember only the first 5 digits of his friendâ€™s phone number. He also knows that the number has 7 digits and that the last digit is not a 0. If Mark were to dial all of the possible numbers and if it takes him 20 seconds to try each one, how long would it take to try every possibility?
3) A contractor is considering a sale that promises a profit of $20,000 with a probability of .7 or a loss (due to bad weather, strikes, and such) of $3,000 with a probability of .3. What is the expected profit?

Alegebra 
Jessi,
this is what i got from the 2nd problem
of the total 7 digits in the number he knows the first 5 that leaves 2 for him to find out with 0 not being the last number so he knows 10 possible for the 6th and 9 possible for the 7th (not 0) 10 * 9 = 90 possible numbers
then it takes 20 seconds to try each one so 90 * 20 = 1800 seconds = 30 minutes
I'm I correct? 
Alegebra 
Jessi,
#3 I got this answer
(20000*.7)(3000*.3)
=14000900
=$13,100. expected profit
Is this right 
Alegebra 
Jessi,
Ok so I really need help with problem 1 I have no clue

Alegebra 
bobpursley,
correct on 2.
correct on 3.
1) what is 1/7 * 1/5 ? 
Alegebra 
Jessi,
So the answer for #1 is 1/35 is that I have to do for that problem though?

Alegebra 
MathMate,
This is how it works.
To draw a 4 out of 7 balls, you have 1 success out of 7 possible events.
To draw a 2 out of 5 balls, you have 1 success out of 5 events possible.
For both of them to happen, you use the rule of multiplication.
See also:
http://www.jiskha.com/display.cgi?id=1252450211