Brain Teasers & Mental Math
A stock falls 50%, then rises 50%. Where does it end up versus the start?
Model answer
Down 25% from the start.
Start at 100. Fall 50% → 50. Rise 50% of 50 → 50 × 1.5 = 75. So 75 vs 100 = a 25% loss.
Key insight: percentage gains and losses are not additive…
The full, human-reviewed answer is in the bank.
Sign up free and Daily 10 serves you 10 questions a day from all 1,500+ — or go Pro for unlimited reps.
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?