TITLE: Multiplying probablity
QUESTION [1 upvotes]: I am trying to help my kid do the following probability math. The language of the math baffles me. If the probability of the pictures tunring out is 1/5 then how can it become 3/4 (howeven numbers are taken) Any help in understanding the problem and a clue to solve it will be appreciated.
When Trilisa takes pictures, they turn out with probability $\frac{1}{5}$. She wants to take enough pictures so that the probability of at least one turning out is at least $\frac{3}{4}$. How few pictures can she take to accomplish this?

REPLY [3 votes]: When a picture turns out with probability $\frac 15$, it means they don't turn out with probability $\frac 45$.
Likewise, saying the probability that at least one turns out is $\frac 34$ is the same as saying that the probability that none turns out is $\frac 14$.
$(\frac 45)^n=\frac 14$ is the equation you have to solve.
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