deloitter wrote:

I'm about 3 months away from my GMAT and my quant is now consistently between 35-40, whereas prior, during the time of my diagnostic, I was at sub 20! Yikes.

I've done about a month of review using

Magoosh. I'm now wondering if it's worth it to start reading books like "Mental Math: Tricks to Become a Human Calculator" by Abhishek VR to increase my mental maths.

Not sure if this will take away energy I could just devote to doing questions. Thoughts?

You don't need

mental math, per se. Writing stuff down isn't bad and doesn't take too much time. It's also good to write down your work because it makes you more likely to catch mistakes as you make them. Mental math books or sites focus more on things that are effectively party tricks. Nobody's watching you perform while you take the GMAT, though.

On the other hand, learning to do arithmetic more quickly is

very useful on the GMAT. Things like multiplying multi-digit numbers on paper (don't go overboard, but you should be able to multiply two-digit numbers very quickly, for instance.) Practicing arithmetic will help you get through problems more quickly, which will give you more 'breathing room' to consider your approach and double check your work.

In short: I wouldn't bother with that book, but I would recommend using a site like Khan Academy (or just googling 'middle school math drills') to practice your arithmetic speed.

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