Choice B is the correct answer. Let $t$ = time in hours.

Analysis: When traveling in opposite directions, the distances add up.

• Train 1 distance: $60t$
• Train 2 distance: $80t$
• Total distance: $60t + 80t = 140t$

Step 1: Set up equation: $140t = 420$

Step 2: Solve for $t$: $t = \frac{420}{140} = 3$

Solution: 3 hours

Verification: In 3 hours, Train 1 travels $60(3) = 180$ miles, Train 2 travels $80(3) = 240$ miles. Total: $180 + 240 = 420$ miles ✓

💡 Strategic Tip: Opposite directions → add speeds. Same direction → subtract speeds.

Choice A is incorrect because in 2 hours: $60(2) + 80(2) = 280 \ eq 420$.

Choice C is incorrect because in 4 hours: $60(4) + 80(4) = 560 \ eq 420$.

Choice D is incorrect because in 5 hours: $60(5) + 80(5) = 700 \ eq 420$.