15.6.7 Definition and properties of probability

Problem: There are black, white and blue balls in three boxes. It's known that in the \( 1^{\text {st }} \) box there are 2 white, 4 blue and 5 black balls, in the \( 2^{\text {nd }} \) box there are 4 white, 3 blue and 5 black balls, and in the \( 3^{\text {rd }} \) box there are 1 black, 2 white and 2 blue balls. From a randomly chosen box 3 balls are taken out. Find the probability that they are of the same colour.