When Trilisa takes pictures, they turn out with probability 1/5. She wants to take enoughpictures so that the probability of at least one turning out is at least 3/4. How few pictures can she take toaccomplish this?
If it 1 has to make it, then we can find the probability of none making it and work from there. If the probability wanted is 3/4, then 1/4 is the chance they all don't make it. So (4/5)^x has to be less than .25.
The smallest number is 7
Each picture she takes has a probability of (4/5) of not turning out
The probability of at least one turning out = 1 - probability that none turn out
So we have
1 - (4/5)^n = 3/4 where n is the number of pictures she needs to take
1 - 3/4 = (4/5)^n
1/4 = ( 4/5)^n take the log of both sides
log (1/4) / = log (4/5)^n
log (.25) = log(.8)^n and we can write
log (.25) = n * log(.8)
n= log (.25)/log (.8) ≈ 6.21
So..as 4M found ......she needs to take 7