Use the Binomial Probability rule, letting X = 5. This denotes the number of heads we want.
[tex]P(X = 5) = \left(\begin{array}{ccc}25\\5\end{array}\right) (\frac{1}{2})^{5} (\frac{1}{2})^{20} = 0.00158 \text{(5dp)}[/tex]
This means that in 25 spaces, we want exactly 5 of these spaces to be heads, leaving the other 20 to be tails (25C5). Now, we just need to find the probability of each event occurring. Since it's a fair coin, there is a 1/2 chance at getting a head, and 1/2 chance at getting a tail.
Since we want 5 heads, we want this event to occur 5 times:
[tex](\frac{1}{2})^{5}[/tex]
and vice-versa for tails.
Thus, we get the answer above.
