Answer
The total number of outcomes is equal to 8 $(n(S) = 8)$
The elements of the event $E$ are: $E = \{ HHT, HTH, THH, HTT, TTH, THT, TTT\}$
And the probability of $E$ is equal to $\frac 7 8$
Work Step by Step
Three coins are tossed.
1. A coin toss can result in Head or Tails, both outcomes with the same probability of occurring, so:
$S = \{HHH, HHT, HTH, THH, HTT, TTH, THT, TTT\}$
Therefore: $n(S) = 8$, which is the total number of outcomes.
2. The set: "E" will have all the outcomes with one or more tails:
$E = \{ HHT, HTH, THH, HTT, TTH, THT, TTT\}$
$n(E) = 7$
3. Calculate the probability:
$P(E) = \frac{n(E)}{n(S)} = \frac 7 8$