Write the unit next to every important number and convert units deliberately before calculating. Do not rely on memory when thousands, millions, months, and years are mixed.

A simple habit is to state the equation with units before doing the arithmetic. That catches many errors before they happen.

At the end, make sure the unit of the answer matches what the question asked for. If useful, this guide on case interview mental math gives more detail.

units next to every number. boring but saves me constantly.

I box the final unit so I don't say annual when calc was monthly.

thousands vs millions is my personal enemy.