We have 9 cards numbered 1 to 9. two cards are drawn at random. what is the probability that the sum of the drawn cards is a prime number less than 9

There are 5 cards less than nine that are prime, so there is a 5/9 chance of picking the first one prime. The second one would be 4/8 or 1/2, because you have removed a card from the deck. You multiply them together to get: 5/18. 

Hope this helps!