In order to know the probability of such an event, we need to know how many cards are red and how many are black, without ambiguity.

(1) tells you about ten cards, but obviously there are ten we know nothing about.

(2) also tells you about ten cards, but once again, there are ten we know nothing about.

(1) and (2) together reveal the necessary information about all twenty cards, and so we may determine the probability (1/2).

This isn't a very difficult question, but as long as you nail it within a reasonable amount of time, you should feel comfortable. Don't feel as though you need to get every "easy" question in thirty seconds, or something.

When I first read it I thought I'd it in 10s.

Then when I thought about it I was incorrect.

May be I was sleepy or something.