Mocahel has 30 small balls of different colors: yellow , green, blue and black. 25 are not yellow, 8 are black and 16 are not green. how many blue balls does michael have?

He has 3 blue balls because if 25 are not yellow it means 5 are yellow and if 16 are not green it means 14 are green balls. If that is the case then add them both plus the number of black balls given (8) then you get 27. The remaining 3 are the blue balls.