A car and a train move in opposite directions; their relative speed is 90 km/h. If the car’s speed is 15 km/h, find the train’s actual speed.

Introduction / Context:
For opposite directions, relative speed equals the sum of magnitudes. Knowing the car’s speed and relative speed, the train’s speed is the difference (relative minus car’s speed).

Given Data / Assumptions:

• v_rel = 90 km/h (opposite).
• v_car = 15 km/h.

Concept / Approach:
Opposite ⇒ v_rel = v_train + v_car ⇒ v_train = v_rel − v_car.

Step-by-Step Solution:

Verification / Alternative check:
Sum check: 75 + 15 = 90 km/h—matches the given relative speed.

Why Other Options Are Wrong:
80 or 60 give sums 95 or 75; 100 or 105 far exceed the given relative speed.

Common Pitfalls:
Subtracting the other way around; confusing same vs opposite direction rules.

Final Answer:
75 km/h