IR questions are bit lengthy and time consuming. So, you need good amount of practice. Try to finish at least 8 questions and make a guess on the others.

Identify your strong and weak areas in this section and act accordingly as which questions need to be solved and which ones are to be guessed.

I hope 5 is a decent score for IR because a score below that might create a doubt in the minds of the admission committee if you have excellent quant score.

I was able to score a 5 on the actual GMAT.

Hope that helps!

Be sure to learn to use both the calculator and the sorting function that is part of tables. Both of them can help you get answers much faster.

Also, realize that you don't need to answer all of the questions and you can even score 7 or 8 with only eight or nine correctly completed questions, depending on their difficulty. So you don't have to worry about answering all 12.

Be careful looking at graphs. Somehow the simple task of noticing what values correspond to the points on the graphs can be a little challenging, and if you are not careful you can easily get the wrong impression.

In fact, being careful and paying attention to details is key in a few ways. For instance, maybe the question asks about percent change when a graph is showing absolute change. Getting the question right will take noticing that.

Practice makes perfect. So find a bunch of IR questions and practice in order to learn what it takes to get them right. GMAC provides some, Veritas has some, and there are plenty more out there from various sources. I have even made a point of guessing on the IR questions I didn't have time to do on practice tests just so I could go back and use them for practice later.

I would be skeptical of the calculator. In my experience, it's mostly there to tempt you to waste time being exact when you could save time making an estimate. (Most of the problems that turn on such estimates have 'friendly' numbers that end up being easy to ballpark.)

The calculator is helpful on some of the two-part word problems, though, I suppose, especially if you're plugging in your own numbers or something.