I was confused with one-third, I thought it is .25\times180 - (1/3) but luckily it isn’t in the answer choices. So, what if it was in the answer choices?
My question is will in the REAL GRE will be told that 1/3 less than the .25\times80 or we memorize or what ? how to differentiate?

No, no never memorize these things…just try to parse the sentence. Go one sentence at a time. Always try to find a meaning of the sentence…

Here, it clearly says 25% of 180…which is 45…Now it mentions 1/3 less than 45…so if 45 is the total or divided in 3 parts…you need to take out 1/3 from it (or 1 part of the three parts)…so you are left with 2/3 of 45…which means it’s 30. Or you can find 1/3 of 45 which is 15…and deduct that from 45.

For me it was the context (question was about percentage) but the more I look at it the more I come to agree towards the conclusion that this question is very poorly written (The author could have easily mentioned find 75% of). So, overall both interpretation \text{X}- \frac{1}{3} \text{&}\text{X}-\frac{X}{3} are valid assumption for the problem.