This is a classic Data Sufficiency question that tests whether you truly understand the relationship between compound and simple interest formulas. I scored 725 on GMAT Focus, and this type of problem used to trip me up until I grasped the key concept.
The question asks: if the same amounts are borrowed at the same rate for three years, by what percentage will compound interest exceed simple interest?
Let me break down the formulas given:
- Compound Interest = P[(1 + r/100)^n - 1]
- Simple Interest = P × r × n / 100
Understanding What We Need:
We need to find what percentage CI is greater than SI. This means we're looking for: [(CI - SI) / SI] × 100%
Analyzing Statement 1: The rate of annual interest is 10%
Here's the critical insight that many students miss: when a Data Sufficiency question asks for a percentage or ratio, you often don't need absolute values because they cancel out.
Let's work through this with r = 10% and n = 3:
- CI = P[(1.1)3 - 1] = P[1.331 - 1] = 0.331P
- SI = P × 10 × 3 / 100 = 0.30P
Now calculate the percentage difference:
- Difference = 0.331P - 0.30P = 0.031P
- Percentage greater = (0.031P / 0.30P) × 100% = 10.33%
Notice how P cancels out! We got a specific answer (approximately 10.3%) using only the rate. Statement 1 is SUFFICIENT.
Analyzing Statement 2: The amount borrowed is $100,000
This gives us P = $100,000, but without knowing r, we cannot calculate either CI or SI. Different interest rates would yield completely different percentage differences. For example:
- At 5% for 3 years: CI exceeds SI by about 5%
- At 10% for 3 years: CI exceeds SI by about 10.3%
- At 20% for 3 years: CI exceeds SI by about 21%
Statement 2 is INSUFFICIENT.
Answer: A (Statement 1 alone is sufficient)
Common Trap Alert: The biggest mistake students make here is thinking they need both P and r to answer the question. This comes from not recognizing that when you're asked for a percentage difference or ratio, the absolute value (principal in this case) often cancels out in the calculation. This is a fundamental Data Sufficiency pattern you'll see repeatedly - whenever the question asks "what percent" or "what ratio," consider whether you actually need the absolute values or just the relative relationship.
Key Takeaway: For percentage-based questions in Data Sufficiency, always ask yourself: "Will the values I'm missing cancel out in my final calculation?" This mental check will save you from eliminating correct answers.