In a bag there are 4 balls.
2 balls are drawn together at random and both are found to be white . what is the probability that all the balls in the bag were white in color.
Here is the answer and you can figure out where all the bits and pieces come from and what assumptions have been made:
$\displaystyle \frac{ \frac{1}{3} \cdot \frac{{4 \choose 2}}{{4 \choose 2}} }{\frac{1}{3} \cdot \frac{{2 \choose 2}}{{4 \choose 2}} + \frac{1}{3} \cdot \frac{{3 \choose 2}}{{4 \choose 2}} + \frac{1}{3} \cdot \frac{{4 \choose 2}}{{4 \choose 2}} }$