Hi "i like c",

there are 2 ways to work it out, if not more...

If a sample of 4 balls are taken,

we can list the order of selection if they are chosen one-by-one.

RRBG red 1st, red 2nd, blue 3rd, green 4th....

RRGB

RBRG

RBGR

RGRB

RGBR

GRRB

GRBR

GBRR

BRRG

BRGR

BGRR

If you calculate the remaining probabilities and sum them, you will have the

probability of choosing 2 reds, 1 blue and 1 green.

Alternatively there are ways to choose 4 from 9.

There are ways to choose 2 reds

There are ways to choose one blue and only 1 way to choose the green.

Any 2 reds can go with any blue, hence we multiply those numbers of ways

to find the total number of ways we can get 2 reds, 1 blue and 1 green

Hence you divide that number by the total number of ways to choose 4 from 9.