How many seconds will a boy take to run one complete round around a square…

Concept: For motion at a constant speed, time = distance ÷ speed. A speed given in kilometres per hour is converted to metres per second by multiplying by 5/18 (equivalently, dividing by 3.6). The distance covered in one complete round of a square field equals its perimeter, 4 × side length.

Application:

1. Find the distance for one round: the field is a square of side 30 m, so its perimeter = 4 × 30 = 120 metres.

2. Convert the given speed to metres per second: 3 km/hr = 3 × 5/18 m/s = 15/18 m/s = 5/6 m/s.

3. Apply time = distance ÷ speed: time = 120 ÷ (5/6) = 120 × 6/5 = 144 seconds.

Cross-check: Working entirely in the original units confirms this: 120 m = 0.12 km, so at 3 km/hr the boy takes 0.12 ÷ 3 = 0.04 hours, and 0.04 hours × 3600 seconds/hour = 144 seconds — the same result.