Several things are important during the calculation phase of the interview:
The following skills will speed up your calculations and reduce the probability of making a mistake:
1) Learn how to solve equations and systems of equations. Some cases require solving an equation. You can use GMAT books to practice.
2) Learn how to multiply double-digit numbers really fast. It takes just a couple of hours to learn how to apply this method on paper and a couple of days to start doing these calculations mentally, but it's worth it. Please follow the link for more details: https://www.youtube.com/watch?v=6ndkkPZYJHo
3) Learn how to work with zeros.
Simply always use the powers of 10. Then you'll be able to multiply / divide the numbers and sum up / subtract the powers separately:
For example: 300 x 9000 = 3 x 10ˆ2 x 9 x 10ˆ3 = 3 x 9 x 10ˆ(2+3) = 27 x 10ˆ5 or 2.7 Mln
If you get used to writing all the numbers that way, you will never lose zeros.
4) Learn the division table. This method will help you calculate any percentage problems like market shares or margins. For example, if your market is $620M and your revenues are $5.1M you can use 5/6 or 83.3%, as a proxy to calculate the market share. By adjusting to zeroes and slightly decreasing the number, you'll get 8.2%