First of all, I recommend using the following approach while doing your calculations
Tell the interviewer your approach / formula
Check with the interviewer that your approach is correct
Ask for a minute to perform the calculations
Check any assumptions you make with the interviewer. If you need to round the number - you have to check with the interviewer as well
Come to the interviewer with some intermediate calculations results to check that you are moving in the right direction
Come up with the final answer
Provide the conclusions from the final answer
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. As you could have noticed, 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%