A urn contains 7 red and 3 black balls. Two balls are draw with out replacement. Find the probability that the second ball is red if it is known that the first is red.

urn contains
no of red balls $-7$ ; no of black balls $-3$   

total$-10$ balls
One ball is drawn then the ball is red The remaining total balls is  $3+6=9.$
The second ball is red we know that the first

ball is red then probability  $=\frac{6}{9}=\frac{2}{3}$