One half of the marbles are red a third are white the other 6 are blue

1/2 + 1/3 = 3/6 + 2/6 = 5/6 ... so 5/6 of the total marbles are red and white.

That means 1/6 are blue. So if 6 are blue, then 1/6 of the total marbles = 6

1/6x = 6

x = 6 * 6/1

x = 36 ... there are 36 total marbles

1/2 are red ... so 1/2 (36) = 36/2 = 18 are red

1/3 are white ... 1/3 (36) = 36/3 = 12 are white

and 6 are blue.