Hey Fadi,
Interesting question...I guess you could go one of two ways with it, so hopefully one of these is what you're thinking:
1) You have 20 seconds or less left and have to just guess without any time to really think.
Read the question and get a feel for how they'd set up the answer choices so that you can strategically rule out the likely "trap" answers. Since they're asking about Bill's Sunday run, we can tell that:
It's longer than his Saturday run
It's shorter than Julia's run
There's a good chance that the GMAT would set up answer choices that would correspond with the Saturday run or Julia's run, so you're looking for a number toward the middle of the range of answer choices (the biggest number may well be Julia's, the smallest may well be Saturday).
Now...that's a pretty low-percentage "get it right" strategy, but definitely not a bad "give yourself a significantly better than 20% blind guess" strategy. Picking a number toward the middle helped you get close to that correct answer of 7, but neither Saturday's nor Julia's run was an answer choice, so the strategy didn't really work as planned.
2) You want to backsolve (not really guess) using the answer choices.
Here, the strategy is to start with a middle number, plug it back into the word problem, and determine whether it's:
Correct (you're done)
Too small (you can eliminate it and anything smaller)
Too big (you can eliminate it and anything bigger)
If you start exactly in the middle, you can plug in C:
7 is the Sunday run, 7*2 = 14 is Julia's run, and 7-4 = 3 is the Saturday run. 7 + 14 + 3 = 24 so that's correct.
However, if C is not right, you have to do one more problem to get it right (say C were too small, you'd have to check D, and either that would be right or if not it's E).
Therefore, if you have a feeling that you need a larger number or a smaller number (say they were asking about the Saturday run, you'd have a good inkling that the trap answers could be the larger numbers for Sunday and Julia), then you can pick B or D to backsolve, with the added benefit that if B is too big, only A is left as a possible choice. This way, you're more likely to have the correct answer in just one step.
All that said, if you find a question relatively straightforward using the algebra, why guess or backsolve? You may well be at the point where the algebra is faster than trying to find a "faster" way...
Brian Galvin
GMAT Instructor
Chief Academic Officer
Veritas Prep
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