GMAT Quantitative Question of the Day

1. $\dfrac{N}{4}$
2. $\dfrac{N}{2}$
3. $2N$
4. $4N$ (correct answer)

Explanation: This is an exponential growth problem where you need to track how a quantity changes over equal time intervals. When you see "doubles every X hours," you're dealing with exponential growth with a base of 2. First, establish the pattern. The bacteria doubles every 3 hours, so:

• At 0 hours: some initial amount
• At 3 hours: 2 × initial amount
• At 6 hours: 4 × initial amount
• At 9 hours: 8 × initial amount = $N$
• At 12 hours: 16 × initial amount
• At 15 hours: 32 × initial amount

• At 12 hours: $2N$
• At 15 hours: $2(2N) = 4N$