A car travels at a steady, unchanging speed

Why

Roshni's maths is exactly correct for a car holding a truly steady speed — distance and time DO scale together proportionally when speed never changes, so 40 km an hour really does mean 120 km in three hours. The one thing worth checking is the assumption she's making explicit: 'provided the speed stays the same.' Real journeys almost never hold one unchanging speed for three whole hours — traffic, red lights, turns, and tiredness all change it along the way, and 40 km covered in the FIRST hour doesn't guarantee the same pace for the next two. If the assumption genuinely holds — and the question does state it does — her arithmetic is right. Test: a car's speedometer on a real three-hour trip rarely sits on one exact number the whole time, which is exactly why real distances rarely land on a neat multiple like this. Yes, Roshni is right, precisely because the question states the speed stays steady — the arithmetic (40 × 3 = 120) only works because that assumption is given, and it's worth noticing how much work that one assumption is doing.