th of them are red, ths or them are blue.

That means ths of them are either red or blue, so

the remaining  have to be yellow, so the probability
of getting a yellow sock is ths.
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Or you could look at it this way:   

Suppose there are R red socks, B blue socks and Y yellow socks.

Then there are R+B+Y socks in the bag.

Since the probability of getting a red sock is ,



Cross-multiplying:





Since there are twice as many blue socks as red socks, 

so if we substitute that:





So there are twice as many yellow socks as red socks,
so you're twice as likely to select a yellow one as a
red one, so twice  is .

Answer: 

Checking: There could be just 5 socks in the bag, 1 red ones,
2 blue ones and 2 yellow ones.  There are twice as many blue
ones than red ones, and the probability of getting a red one
is , and the probability of getting a blue one is the
sam as that of getting a yellow one, which is is {{2/5}}}.

Edwin