Can you calculate the probability of getting one head and one tail?                                      [Page No. 169]

When two coins are tossed, the sample space is $S = \{HH, HT, TH, TT\}$ with four equally likely outcomes.
'One head and one tail' happens in two of these: $HT$ (first coin head, second tail) and $TH$ (first coin tail, second head).
Both show exactly one head and one tail, so there are $2$ favourable outcomes out of $4$.

Therefore

$P(\text{one head and one tail}) = \frac{\text{favourable outcomes}}{\text{total outcomes}}$

$$= \frac{2}{4} = \frac{1}{2} = 0.5 \text{ or } 50\%.$$

A common slip is to think there is only one way to get 'a head and a tail'. But HT and TH are different outcomes, they differ in which coin shows the head even though both have one head and one tail. That is why the probability is $\frac{1}{2}$, not $\frac{1}{4}$.