Aside from manipulating the GMAT’s favorite numbers (72, 64, etc.), one of the calculations you’ll do most often when working through GMAT problems is dividing and multiplying by five. It’s common in the real world, too. As with most common calculations, there is a better way to do them than long division or traditional multiplication.
For both division and multiplication, the key concept here is that 5 is simply 10 divided by 2. So, anywhere you see a 5 in an equation, you can substitute (10/2). You won’t always want to do that, but in some cases, I guarantee you that working with 10s and 2s is preferable to working with 5s.
Using that trick, consider multiplying 36 and 5. (If you automatically know that, work through the example with a less common number, like 47.) Using the trick outlined above, 36(5) = 36(10/2). You now have two options: you can multiply 36 and 10 and then divide by 2, or divide 36 by 2 and then multiply by 10. Either way, both steps are quite simple.