It's ok to write down your calculations
Of course you can make it on paper. You have to do it to make you calculations clear and communicapable.
Consulting math is a very different than academic math. Working consultants - and consulting interview candidates - are always under time pressure. Results are what matter and answers are required simply to be good enough to guide business decisions, rather than being absolutely correct.
Also remember here about estimating and adjusting.
For example: 96,840/529
Step 1: Estimating
We will have a simplified version of: 96.84/52.9, taking out 3 zeros in the numerator and 1 zero in the denominator which means that we will return 2 zeros in the last step.
Technically we need a step of “rounding” here. But in this case, we can just skip it and make an overall adjustment later.
Now turn division into multiplication and find x:
52.9 x ? = ~96.8
We know 53 x 2 = 106
Thus, ? is a little bit lower than 2.
Step 2: Adjusting
With a downward adjustment for 2, we have 1.8. And with zeros, it is 180.
The correct answer is 183.06
Hope it helps,