Brain Teasers & Mental Math
An investment compounds at 6% per year. Approximately how many times does it multiply over 36 years?
Model answer
About 8× (roughly 2³).
Rule of 72: doubling time ≈ 72 / 6 = 12 years. Over 36 years that is 36 / 12 = 3 doublings, so 2³ = 8×.
Check: 1.06^36 ≈…
The full, human-reviewed answer is in the bank.
Sign up free and Daily 10 serves you 10 questions a day from all 2,300+ — or go Pro for unlimited reps.
Pro unlocks every model answer — $19.99/mo.
More from Brain Teasers & Mental Math
- You roll one fair six-sided die. What is the probability you roll a number greater than 4?
- Flip a fair coin twice. What is the probability of getting at least one head?
- Draw one card from a standard 52-card deck. What is the probability it is a face card (J, Q, or K)?
- Draw one card from a 52-card deck. What is the probability it is a heart OR a king?
- Roll two fair dice. What is the probability the two dice show the same number (a double)?
- A test for a disease is 99% accurate (99% sensitivity and 99% specificity). The disease affects 1 in 10,000 people. You test positive. What is the probability you actually have it?
Try the real thing
1 / 3M&A & Merger Models
What makes an acquisition accretive or dilutive to EPS?
Superday coming up? Take the cheat sheet.
The technicals and stories to have cold before you walk in — free, one email, no mailing list.
Free tier: 20 cards + 10 daily reps + ALL drills — no credit card required.