Hi chacha212.
I am going to show you how to do this one, but before you read my explanation, maybe you should again try to do this problem on your own. This one is not that difficult and maybe part of preparing for the GMAT is about learning to wrestle with problems until you figure out the answer.
We know that there are 12 blue balls. What we are seeking to achieve is a less than 2/5 chance that a single ball drawn from the bag will be blue.
To get a 2/5 chance of a single drawn ball being blue, we would need the number of blue balls, B, to be 2/5 of the total, T.
B = 12 12/(2/5) = 12 x 5/2 = 30
Ok so if the total were 30, in drawing a single ball, we would have a 2/5 chance of getting a blue ball.
Now to get to the answer, we want to make that chance less than 2/5.
How do we achieve that? Well we are stuck with 12 blue balls. So we can reduce the chance of getting a blue ball by making the total higher. The minimum greater total is 31. If the total is 31, the chance of drawing a blue ball becomes slightly less than 2/5.
So we have T = 31 and B = 12 To get the number of yellow, Y, T - B = Y = 31 - 12 = 19
Choose C.
