Find the number of ways of selecting 5 balls from a bag containing 15 balls,in which 6 balls are blue, 6 balls are red and other 3 balls are white,green,yellow.

Three cases can be there-
All three balls are of different color, 2 of same kind and 1 different, 3 of same kind.
There are 5 kinds of balls- Blue-6, Red-6, white, green, yellow.
All three different-$$ ^{5}\textrm{C}_{3}=10$$
2 of same kind and 1 different- $$ ^{2}\textrm{C}_{1}\times^{4}\textrm{C}_{1}=8$$
3 of same kind- only 2 cases possible.
Total number of ways is 10+8+2=20.