Probability and relative frequency

Probability on the SAT is counting, not theory. Almost every question is and the difficulty lies entirely in deciding which numbers those are.

The data nearly always arrives as a two-way table, and the trick is reading which part of the table the question restricts you to.

If a question asks for the probability that a randomly chosen student takes the bus, the denominator is everyone: $55/100$. But if it asks for the probability given that the student is in Year 11, you are confined to the Year 11 row, so it is $32/50$.

That word given is the whole question. It tells you to shrink the denominator from the grand total to a single row or column. Missing it is the most common error in this topic.

Relative frequency is the same calculation applied to observed data rather than theory: the proportion of times something actually happened. The SAT uses it to ask whether a sample's results can be extended to a wider population, which they can only when the sample was selected at random.

Using the table above, what is the probability a randomly chosen student travels by car?
The denominator is the grand total, because no restriction was given

In a random sample of $200$ shoppers, $46$ bought fruit. The shop serves $5000$ customers a week.
Estimate how many buy fruit.
The relative frequency is $\frac{46}{200} = 0.23$, and because the sample was random it can be applied to the whole population Had the sample not been random, no such estimate would be justified — and that is usually the correct answer choice when it appears.