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A company has $300M EBITDA growing at 15% per year. What's EBITDA in 5 years (mental math)?
Answer
Use the Rule of 72: years to double = 72 / growth rate = 72 / 15 ≈ 4.8 years. EBITDA roughly doubles in 5 years → ~$600M. (More precisely: $300 × 1.15^5 = $300 × 2.011 ≈ $603M.)
Why interviewers ask this
Rule of 72 is essential mental math for any finance role. Variants: Rule of 114 (years to triple) and Rule of 144 (years to quadruple). Memorize: 10% doubles in ~7yr, 15% in ~5yr, 20% in ~3.5yr.
growthrule of 72mental math