A box contains red balls, white balls, and yellow balls. If a ball is

Hi Mike,
Indeed the "1minus the opposite" was mainly the trick I was referring to.

But also, I think that you can get confused because the problem does not actually give you the number of the balls - so we don't know how many they are. So, initially, one might think that, oh, none of the 2 data gives me the number of balls, so I cannot possibly answer this question. Then, being in a hurry anyway, you may disregard the second data point and move on.

Also, the first data point gives you 1/2, which is 50% and the second one refers to the second of the 3 colours. This may incorrectly lead you to assume that you know how many half of the balls are. The rest is the other 50%. However, we have three colours here and the data points only refer to 2. Your poor, tired eye, having seen the 50% and reading clues about 2 of the three colours, might forget that the question stem actually mentioned 3 colours.

Or is it only me...