The problem asks for the maximum distance a three-wheeled vehicle can travel when it has 4 new tyres available (3 mounted + 1 spare), and each tyre has a maximum usage limit of 90 km.
To maximize the total distance, we need to utilize all 4 tyres effectively. The vehicle uses 3 tyres at any given time. The core idea is to rotate the tyres, replacing a worn tyre with a less worn one (including the spare) to ensure no tyre exceeds its 90 km limit while covering the maximum possible ground.
We can think about the total "tyre-kilometres" available and required.
$3 \times D = 4 \times 90 \text{ km}$
$3D = 360 \text{ km}$
$D = \frac{360 \text{ km}}{3}$
$D = 120 \text{ km}$
By rotating the 4 available tyres, the three-wheeled vehicle can cover a maximum distance of 120 km.
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