Total number of marbles in the bag =20
Number of blue marbles =4
Now we just want two blue marbles out of four.
Probability of taking out first marble which is blue =4/20
Now after we took out one blue marble , number of blue marbles left are 3.
Probability of taking out second marble which is blue =3/20
Total probability = 4/20 *3/20 = 12/400
So probability of taking out two blue marbles is 0.03 or 12/400.
