How do I get better at the mental math needed for MOIC / accounting problems?

I’m prepping for upcoming SA IB interviews and have found myself really struggling with some of the math needed for accounting and LBO questions. For example, when you need to calculate the interest shield on something, you often have to do 40% of a whole number or 20% of some whole number. Like knowing that 40% of 75 is 30 is not intuitive to me. Same issue for add backs on the CFS.

Do interviewers expect you to do this pretty fast? I don’t come from a math background, so sometimes I need to write it out - which can take me up to 90 seconds. I assume this wouldn't fly during an interview.

Also, for MOIC (knowing that 1200/500 = 2.4, for example), I have the same issue. Did anyone else run into this in recruiting?

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