combination

Hello, prasum!

This one is very easy.
Exactly where is your difficulty?

Quote:

$\text{There are 24 balls of 6 different sizes in a bag, there being 4 balls of each}$
$\text{size in four different colours. \:In how many ways 4 balls can be selected }$
$\text{so that they are of different colours?}$

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: . $R_1,\:R_2,\:R_3,\:R_4,\:R_5,\:R_6$

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: . $6^4 = 1296$ ways.