Don't worry about this! You will rarely have to do complex calculations in your head. Consultants don't need to do this.

Instead, what I'd focus on is that at a 25% growth rate, you'll double in approximately 3 years (due to rule of 72) and then add the same amount again in the next 2 years approximately (due compound growth). In summary, therefore, you can triple in 5 years, so the answer should be around 3. (In reality, when I calculate electronically, the answer is 3.05)

Does this help?

Allen has an excellent approach! The goal of math in casing is not to see if you can multiple super hard numbers together like an engineer. The goal is generally, can you figure out quick-and-dirty approaches to complicated situations, to get to the general answer that matters.

Hi, here you can find some good approaches to quickly crack it: https://www.preplounge.com/en/consulting-forum/estimate-compounding-4962

Best

Antonello

Hello! I agree that you do not need to make complex calculations in your head and that your approach to solve difficult problems is more important.
Hope it helps,
All the best
GB

(edited)