ratio problem

A

R:W=1:3

B

R:W=1:4

Note that white marbles in B are in multiples of 4.

So possible scenarios are A=18,B=12 or A=6,B=24

If white marble in A is 18,then red marble is 6

If white marble in A is 6,then red marble is 2

Only 6 is available in option .

Hence D

Plugging values is also easy in this case.

Red marbles in a (Ra) + Red Marbles in b (Rb) = 30. Let white marbles in bag b = Wb
Also, In bag B, Red:White = 1:4 which means Number of red marbles must be a multiple of 4 (you can't have a fraction of a marble).

1) If Ra = 1, Rb = 29, => Wb = 29/4 => incorrect
2) Ra = 3, Rb = 27 , => Wb = 27/4 => incorrect
3) Ra = 4, Rb = 26, => Wb = 26/4 => incorrect
4) Ra = 6, Rb = 24 => Wb = 4, possibly correct
5) Ra = 8, Rb = 22 , incorrect

Only one option satisfies the ratio, so answer should be (D)