Aryan says a speed of 5 m/s is less than a speed of 10 km/h, because 5 is smaller than 10

Why

Aryan is comparing the numbers 5 and 10 directly, but a speed is a number AND a unit, and the two speeds here are measured in completely different units — metres per second versus kilometres per hour. To compare them fairly, you have to convert one so both are in the same units. 10 km/h means 10,000 metres every 3,600 seconds, which works out to about 2.8 metres per second. Test it: line up the converted values — 5 m/s versus 2.8 m/s — and now 5 is clearly the bigger one, so 5 m/s is the faster speed, the opposite of what the raw numbers suggested. His sister is right: convert to the same units first — 10 km/h is only about 2.8 m/s, so 5 m/s is actually faster; the bare numbers mislead when the units differ.