Out of first 20 natural numbers, one number is selected at random. The probability that it is either an even number or a prime number is -

$$\eqalign{ & n\left( S \right) = 20 \cr & n\left( {{\text{Even no}}} \right) = 10 = n\left( E \right) \cr & n\left( {{\text{Prime no}}} \right) = 8 = n\left( P \right) \cr & P\left( {E \cup P} \right) = \frac{{10}}{{20}} + \frac{8}{{20}} - \frac{1}{{20}} \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, = \frac{{17}}{{20}} \cr} $$