GRE General TestQuantitative ReasoningMedium
A biologist is studying a cell culture experiment. The number of cells triples every 2 hours. If there are initially 500 cells, how many cells will there be after 8 hours?
- A4,000
- B121,500
- C13,500
- D40,500
Show answer & explanationAnswer & explanation
Correct answer: D. 40,500
Determine the number of 2-hour intervals in 8 hours. Then, multiply the initial number of cells by 3 for each interval.
Why the other options are wrong
- A. This is 500 * 8, a linear growth calculation, not exponential.
- B. This results from an extra multiplication by 3 or other calculation error.
- C. This results from multiplying by 3 only 3 times (6 hours), not 4 times (8 hours).
Exponential Growth (Discrete)
Exponential growth describes a quantity that increases by a constant factor over equal time intervals.
- Formula: N(t) = N0 * (factor)^(t/T_interval)
- N0 is initial quantity, factor is growth multiplier, t is total time, T_interval is time for one growth cycle.
- In this case, factor is 3, T_interval is 2 hours.
Memory trick: Intervals Count, Factor Power: Find how many times it grows, then raise the factor to that power.