This is a classic System of Two Linear Equations question — one of the most reliable question types on the GMAT Focus Problem Solving section.
The big trap here: students try to find Vane's March earnings by guessing or making assumptions about her commission rate, instead of actually solving the system. The two-equation setup completely determines both unknowns.
Let S = fixed monthly salary, c = commission per sale.
Step 1 — Set up the two equations from January and February:
January: S + 90c = 2,610
February: S + 45c = 2,205
Step 2 — Subtract the February equation from the January equation to eliminate S:
(S + 90c) − (S + 45c) = 2,610 − 2,205
45c = 405
c = 9
So each sale earns Vane $9 in commission.
Step 3 — Substitute back to find S:
S + 45(9) = 2,205
S + 405 = 2,205
S = 1,800
Step 4 — Find March sales using the average:
Average over Jan, Feb, March = 165 sales
Total sales = 165 × 3 = 495
March sales = 495 − 90 − 45 = 360
Step 5 — Calculate March earnings:
March = 1,800 + 360(9) = 1,800 + 3,240 = $5,040
Step 6 — Calculate the difference:
Jan + Feb combined = 2,610 + 2,205 = $4,815
March − (Jan + Feb) = 5,040 − 4,815 = $225
The answer is B.
Common trap: Students often compute Jan earnings and Feb earnings separately, find the difference between March and each month, and end up picking $405 (answer D). The question asks how much MORE March earnings are compared to Jan AND Feb COMBINED.
Takeaway: Any time a problem gives you two data points with the same fixed component, subtract the equations immediately — it cancels the fixed salary and isolates the commission rate cleanly.