You can use preplounge tool, Viktor Cheng tool, Mimir Math on iOS
Also, I recommend practicing the following:
- Learn how to multiply double digit numbers (https://www.youtube.com/watch?v=6ndkkPZYJHo)
- Learn the division table up to 1/11 (i.e. 5/6 = 83.3)
- Learn how to work with zeros using powers (E.g: 4000000 = 4*10ˆ6)
Good luck with your interviews!