Great work poojaarora1818! Your solution is spot-on. Let me add an alternative approach that might help others see the pattern more clearly.
This is a classic **Compound Interest comparison problem**—a favorite GMAT topic that tests whether you understand the mechanics of different compounding frequencies.
**The Key Insight:**
The question is really asking: "What's the DIFFERENCE in interest earned between two scenarios with the same principal and time period but different rates and compounding frequencies?"
**Step 1: Find the principal from the first scenario**
8% quarterly for 6 months means:
- Rate per quarter = 8%/4 = 2%
- Number of quarters = 6 months = 2 quarters
- Multiplier = (1.02)2
Interest earned = $808
So: P × (1.02)2 - P = 808
P × 0.0404 = 808
P = $20,000
**Step 2: Calculate interest in the second scenario**
12% semiannually for 6 months means:
- Rate per half-year = 12%/2 = 6%
- Number of half-years = 6 months = 1 period
- Multiplier = (1.06)1
Interest = 20,000 × 0.06 = $1,200
**Step 3: Find the difference**
$1,200 - $808 = **$392** → Answer B
**Common Trap:** Many students forget that "6 months" means different things for quarterly vs. semiannual compounding. With quarterly, you get 2 full compounding periods. With semiannual, you only get 1. Missing this detail leads to wrong calculations.
**Takeaway:** On compound interest problems with different frequencies, always convert the time period to the NUMBER OF COMPOUNDING PERIODS first—don't just plug numbers into formulas. The GMAT loves testing whether you truly understand what "quarterly" and "semiannually" mean in practice.