Data Sufficiency – Sales & Percentage Increase Question: How many gift boxes were sold on Monday? Statements: I. Monday's sales were 10% more than the number of boxes sold on Sunday. II. Every third visitor purchased a box, and there were 1,500 visitors on Sunday.

Introduction / Context:
This problem blends ratio-based conversion (visitors to buyers) with a percentage increase across days. We must check if one or both statements provide enough data to compute Monday's exact sales.

Given Data / Assumptions:

• I: Monday = Sunday * (1 + 10%).
• II: On Sunday, 1 out of every 3 visitors bought a box; Sunday had 1,500 visitors.
• Purchasing rule is precise and constant for Sunday as stated.

Concept / Approach:
Statement I relates Monday to Sunday by a fixed percentage. Statement II allows us to convert Sunday's visitors into Sunday's boxes sold. Together, these fully determine Monday’s sales.

Step-by-Step Solution:
From II: Sunday buyers = 1,500 / 3 = 500 boxes. From I: Monday buyers = Sunday buyers * 1.10 = 500 * 1.10 = 550 boxes. Thus Monday's sales are uniquely determined using both statements.

Verification / Alternative check:
I alone: We know Monday is 10% over Sunday but do not know Sunday's count; insufficient. II alone: We can compute Sunday's sales (500), but Monday's percentage relation is unknown; insufficient.

Why Other Options Are Wrong:
Any option claiming sufficiency of a single statement is incorrect because each alone leaves a missing link.

Common Pitfalls:
Forgetting to convert visitors to buyers exactly as stated (every third visitor). Applying the 10% to visitors instead of to Sunday’s boxes sold.

Final Answer:
Both I and II are sufficient