Question 990556
Instead of calculating the actual probability we will first calculate the complementary probability which is the probability of drawing 0 black cards or only red cards. After we have found this complementary probability we will reduce it from 1 and thus get the required probability. 

If we are drawing two cards without replacement, the total number of ways to draw cards = C(52,2)

There are a total of 26 red cards and 26 black cards.
The number of ways to draw only red cards = C(26,2)

P(only red cards are drawn) = C(26,2)/C(52,2)= 25/102

P(at least one black card is drawn)= 1-P(only red cards are drawn) 

P(at least one black card is drawn)= 1-25/102 = 77/102 = 0.75