for many who have difficulty with mental arithmetic, it's just the stressful interview situation. What about you? How good are you at mental arithmetic if you practice alone at home?
If it's just nervousness, I would recommend you practice the interview situation as often as possible (e.g. with friends or on Preplounge).
If it's the mental arithmetic itself, you can practice very well here: https://www.preplounge.com/de/mental-math.php
If there is no other way, you can also do the calculations in writing on paper. However, I wouldn't advise you to memorize any multiplication tables.
If you have further questions, please contact me!