how to find probability of two coins Rudolph flips two fair coins, each with a half probability of getting heads. 
What is the probability he flips two tails in two tosses?
So I was thinking $0.5 + 0.5 = 1$, but I got it wrong.
I guess it doesn't make sense, because he could flip heads and then flip tails, or flip tails then flip heads, or flip two heads.
How do I find the probability? Do I subtract, or divide?
SAT Math.
 A: Denote by $T$ the event "get tail in 1 flip" and $H$ the head event. Then $\mathbb{P}(T)= \mathbb{P} (H)=1/2$. 
When you flip a fair coin two times, the possible outcomes are
$TT$, $TH$, $HT$,  $HH$ and hence the probability of get two tails in two flips, that is the number of favourable outcome over the number of all the possible outcomes, is
$$\mathbb{P} (TT) = \frac{1}{4}.$$
Notice that it is equal to $\mathbb{P}(T)\cdot \mathbb{P}(T) $, why?
A: When you list possibilities for two coins, note that there are four: HH, HT, TH and TT. Each is equally likely, and one of them is the desired outcome: TT.  Thus we have a probability of $\frac14$.
To obtain this from $\frac12$, the probability of obtaining tails on one flip, you would want to multiply, not add: $\frac12\times\frac12=\frac14$. In general, if the probability of one event is $p$, and the probability of a second, independent event is $q$, then the probability that both happen is $p\times q$.
A: probability of head = 0.5
probability of tail = 0.5
probability is independent $\therefore$ probability of two tails $= 0.5*0.5 = {0.5}^2 = 0.25$
