Key concept: Data Sufficiency — Distance and Speed (Two-object meeting problem).
Setting up the algebra correctly is everything on this type of question. The classic trap is thinking Statement 1 alone is sufficient by confusing "they meet halfway" with a direct speed relationship.
Since they meet exactly halfway, both trains travel exactly 60 miles by the time they meet.
Let freight speed = x mph, passenger speed = y mph.
Time for freight to reach midpoint = 60/x hours.
Time for passenger to reach midpoint = 60/y hours.
Step 1 — Evaluate Statement 1 alone: "Passenger leaves 30 minutes after freight."
The freight leaves first, so it travels for 30 extra minutes (0.5 hours) before the passenger even starts.
Equation: 60/x = 60/y + 0.5
One equation, two unknowns. INSUFFICIENT.
Step 2 — Evaluate Statement 2 alone: "Passenger is 6 mph faster than freight."
So y = x + 6. But we have no timing relationship. We know both travel 60 miles, but we cannot determine how long either was traveling without knowing one speed. INSUFFICIENT.
Step 3 — Combine both statements:
From S2: y = x + 6.
Substitute into S1: 60/x = 60/(x+6) + 0.5
Multiply through to clear denominators:
60(x+6) - 60x = 0.5 * x(x+6)
360 = 0.5x^2 + 3x
x^2 + 6x - 720 = 0
(x + 30)(x - 24) = 0
x = 24 (rejecting x = -30 as speed must be positive)
So freight = 24 mph, passenger = 30 mph. SUFFICIENT.
Answer: C
Common trap: Many students pick A, thinking S1 alone gives a direct speed from the 60-mile constraint. It doesn't — you have two separate variables and only one equation. You need both pieces to pin down actual speeds.
Takeaway: In DS distance-meeting problems, always count your equations versus your unknowns before concluding "sufficient."
— Kavya | GMAT Focus 725 (99th percentile)