How do you develop number sense in GMAT ?

The most common method is to choose numbers that are either common multiples or common factors of the numbers in the problem, or, if it's a percent question, to pick 100 for some value. Some percent questions have decimals in percents (e.g. '50.5%') and 1,000 is a better pick.

But if I have fractions of 'thirds' or 'fourths,' I might pick 12 for some total. '60' tends to be a convenient number, as it has many factors. For this question: https://gmatclub.com/forum/working-simu ... 43705.html

we have machines making 800 nails in different hours, so it's probably easiest to pick factors of 800. I wouldn't want to say '800 nails in 3 hours,' because that doesn't divide nicely. 800 in 2, 4, 5, 8, 10... etc. hours makes for much nicer numbers.

You should try, if you can, to plan ahead--think what you'll need to do with the number(s) you pick so that you'll have a sense for what kind of number you want.

If it's a question that has only relative values (all fractions/ratios/percents/proportions), you can pick any numbers that fit those relationships (including just the numbers in the fractions, if it works for all the values in the problem). But scaling up a multiple or two (picking 6, 9, 12, 15, etc. instead of '3') is often good advice (again, look to see what other numbers might be in the problem).

Here is a video about choosing numbers in problem solving that might help: https://www.youtube.com/watch?v=G8DH_sUHHiE

try to read through the answer explanation, as well as the discussions in this forum, and think about how you can replicate the thought process. also think about which method to solve the problem is the easiest/fastest/realistically usable to you on exam day.
next time you come across the same problem, try to remember and use that new approach (this is why doing the same problems 3 - 5+ times is important).

with respect to plugging numbers, just enter 3. Wait, is plugging in an odd number appropriate in the case? Is the number too small? that is the thought process.
When a large number/fractions are mentioned, I usually use 120, as it can be divided by many numbers.

How do you choose those particular numbers which can help you to eliminate answer choices under 2 mins?

This may be a nice video to look through!

How to get better at GMAT Quant. Pick numbers you can actually use.