\(Probability = \frac{Number of outcomes}{Total numbers of outcomes}\)
We know that a coin has 2 sides so total number of outcomes = 2
And we also know that a coin has 1 head and 1 tail side
So,
probability ( getting tails on top ) = \(\frac{1}{2}\)
Your final answer is \(\frac{1}{2}\) or 50% or 0.5