The question is rather ... short. You are probably referring to the Regular Exponential Class (REC). A density function is said to be a member of REC if it can be decomposed as follows (LaTeX code, since I don't know how to write this stuff here):
f(x;s)=c(s)h(x)exp[\sum_{j=1}^{k} q_j(s) t_j(x)] for x \in A and zero otherwise. Here, s is a vector of k unknown parameters (s_1, ... , s_k). Moreover, the following conditions need to be satisfied: the parameter space needs to be compact (i.e. for all i, we have fore each parameter that a_i \leq s_i \leq b_i, for a_i and b_i constants, where also infinity is allowed), also, the set A over which the density is strictly positive should not depend on the parameters s, the functions q_j should be non trivial, functionally independent, and continuous functions of the parameter, and the last condition is that the derivatives t'_j(x) are linearly independent continuous functions of x over A (with a similar condition existing for discrete random variables).
Why we care about REC? Various reasons: e.g. we can then immediately derive a set of complete and sufficient statistics, or construct uniformly most powerful tests for one-sided hypothesis testing.
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The probability is always a fraction except when it is 0 or 1. If a probability = 1 then it will definitely happen. If the probability is 0 then it will not happen. If you toss a fair coin the probability of heads is 1/2, and the probability of tails is 1/2. These fractions are representations of the probabilities. Not all fractions are representative of probabilities. Fractions can be used to represent a portion of a whole. Like what portion of a class is boys, and what portion is girls: If there are 8 boys and 7 girls, then the 8/15 of the class is boys, and 7/15 of the class is girls.
The probability of this is based heavily on whether or not said best friend is even in the class. If both are in, it's a 1/870 chance.Ê
Probability that a girl is chosen = 23/45 = .511 So, the probability that a boy is chosen = 1 - .511 = .489
class width times frequency density gives you the frequency
.9^27, or approximately .058 = 5.8%