A box has 7 white balls,9 red balls and 5 green balls, At least how many balls would you have to take in order to ensure that you have 3 balls of same colour?
This known as the "pigeonhole principle"
By taking 7 + 5 + 3 = 15 balls you are guaranteed to end up with 3 red balls......
Where does it say that they all have to be red ?
If you take six balls, you will either have three the same colour amongst them or three sets of two.
The seventh ball must then give you a set of three the same colour.