there are 24 balls of 6 different sizes in a bag there being 4 balls of each size in four different colours in how many ways 4 balls can be selected so that they are of different colours
This one is very easy.
Exactly where is your difficulty?
Perhaps the idea of six sizes confused you.
This was only to show that the six red balls are distinguishable.
. . They could have been simply numbered: .
There are 6 choices for a red ball.
There are 6 choices for a yellow ball.
There are 6 choices for a blue ball.
There are 6 choices for a green ball.
Therefore, there are: . ways.