GRE General TestQuantitative ReasoningMedium
A scientist is observing the decay of a radioactive substance. The substance decays by half every 4 hours. If an initial sample has a mass of 320 grams, what will be its mass after 12 hours?
- A20 grams
- B40 grams
- C80 grams
- D160 grams
Show answer & explanationAnswer & explanation
Correct answer: B. 40 grams
Calculate the number of half-life periods that occur in 12 hours. Then, divide the initial mass by 2 for each half-life period.
Why the other options are wrong
- A. This would be the mass after 4 half-lives (16 hours).
- C. This would be the mass after 2 half-lives (8 hours).
- D. This would be the mass after 1 half-life (4 hours).
Exponential Decay (Half-Life)
Exponential decay, specifically half-life, describes the time it takes for a quantity to reduce to half of its initial value, occurring repeatedly over fixed time intervals.
- Quantity is repeatedly divided by 2.
- Half-life is the duration of one decay period.
- Used in radioactive decay, drug metabolism, etc.
Memory trick: Half-Life's Hops: Count the half-life periods, then keep dividing by two!