When a fair coin is tossed it will either come up heads or it wont. Put another way, the probability of getting heads is 1 or 0, unless you believe that something magical happens to the coin while it is in mid-air that somehow makes it impossible to predict what will happen when it lands (given a complete set of data).
So the outcome of the coin-toss is knowable, it just happens to be complex enough that it is not known (to us). To bridge the gap between the knowable and the not known we introduce a margin of error in our calculations, called Probability. Probability then, only exists as man-made mathematical spackle - and is by definition subjective.
To prove me wrong find an event that has an outcome that cannot be known in advance. That's why this question is on a philosophy exam.