Concept:
If the odds in favor of an event $E$ are $a:b$, the probability of the event is $P(E) = \frac{a}{a+b}$.
If the odds against an event $E$ are $a:b$, the probability of the event is $P(E) = \frac{b}{a+b}$.
When exactly one of a set of mutually exclusive and exhaustive events must occur, the sum of their probabilities equals 1.
Step 1: Calculate the probabilities of events A and B.
The odds in favor of A are given as $4:6$.
$$P(A) = \frac{4}{4 + 6} = \frac{4}{10}$$
The odds against B are given as $7:3$.
$$P(B) = \frac{3}{7 + 3} = \frac{3}{10}$$
Step 2: Find the probability of event C.
Since exactly one of the events A, B, or C must happen, they are mutually exclusive and exhaustive. Therefore, the sum of their probabilities must be 1.
$$P(A) + P(B) + P(C) = 1$$
Substituting the known values:
$$\frac{4}{10} + \frac{3}{10} + P(C) = 1$$
$$\frac{7}{10} + P(C) = 1$$
$$P(C) = 1 - \frac{7}{10} = \frac{3}{10}$$
Step 3: Determine the odds against C.
To find the odds against C, we first need the probability of C not happening, which is $P(C')$.
$$P(C') = 1 - P(C) = 1 - \frac{3}{10} = \frac{7}{10}$$
The odds against C are the ratio of the probability of it not happening to the probability of it happening:
$$\text{Odds against } C = P(C') : P(C)$$
$$\text{Odds against } C = \frac{7}{10} : \frac{3}{10}$$
$$\text{Odds against } C = 7 : 3$$