Solution: Let $ x $ be the total GB used. The cost equation is $ 12 \times 20 + 0.08(x - 20) = 38.40 $. Simplify: $ 240 + 0.08x - 1.6 = 38.40 $. Combine terms: $ 238.4 + 0.08x = 38.40 $. Subtract 238.4: $ 0.08x = -199.6 $. Divide by 0.08: $ x = -2487.5 $. This negative value indicates an inconsistency, suggesting the business used fewer than 20 GB. Recalculate without the \$0.08 overage: $ 12x = 38.40 $ → $ x = 3.2 $. Verify: \$12 × 3.2 = \$38.40. Final answer: $ \boxed{3.2} $ GB.

["How Businesses Calculate Data Usage Costs: Understanding Overage Fees and Correct Payment Models", "When managing data plans, clarity in how costs are calculated is essential for accurate budgeting and avoiding surprises. A common formula used by service providers estimates total charges based on expected usage. However, mismatches between assumptions and actual consumption can lead to confusing or incorrect billing.", "Consider the equation modeling monthly data fees:\n$ 12 \ imes 20 + 0.08(x - 20) = 38.40 $\nThis represents a fixed monthly fee of $240 for the first 20 GB, plus a $0.08 overage charge per GB beyond 20, totaling $38.40.", "Simplifying step-by-step:\nFirst, expand the expression:\n$$\n240 + 0.08(x - 20) = 38.40\n$$\nDistribute the 0.08:\n$$\n240 + 0.08x - 1.6 = 38.40\n$$\nCombine constant terms:\n$$\n238.40 + 0.08x = 38.40\n$$\nSubtract 238.4 from both sides:\n$$\n0.08x = 38.40 - 238.40 = -199.60\n$$\nDivide by 0.08:\n$$\nx = \frac{-199.60}{0.08} = -2487.5\n$$\nThis negative result indicates a fundamental inconsistency — it suggests the customer used negative GB, which is impossible. The root issue lies in the model assuming charges apply beyond 20 GB when actual usage was under the 20 GB threshold.", "Let’s correct the interpretation by removing the overage fee assumption: if the total cost is simply $38.40 and there’s no overage beyond 20 GB, solving $ 12x = 38.40 $ yields:\n$$\nx = \frac{38.40}{12} = 3.2\n$$\nVerifying: charging $12 for each full 20 GB block implies the plan is structured in increments — but here, $38.40 divided evenly by $12 corresponds to 3.2 GB, consistent with a rate per gigabyte (not per 20 GB).", "Thus, the correct interpretation — aligning cost with actual usage under the plan’s base pricing — is that the customer used 3.2 GB, consuming within the base tier without triggering overage fees.", "Final Thoughts\nThis case highlights how precise cost modeling prevents billing anomalies and supports transparent communication between providers and users. Always verify whether multiply-tier structures or flat per-GB rates apply, and understand usage tiers to avoid miscalculations.", "Final Answer:\n$$\n\boxed{3.2} \ ext{ GB}\n$$"]