Even sum of two different primes up to 20

Two different primes are chosen uniformly at random from the primes in $\{1,2,\dots,20\}$. What is the probability that they add up to an even number?

$\tfrac34$. There are $8$ such primes, and every one except $2$ is odd. The sum is even exactly when $2$ is not chosen, which happens with probability $\binom72/\binom82=\tfrac{21}{28}=\tfrac34$.