4 A bus travels between two stops, \(A\) and \(B\). The bus starts from rest at \(A\) and accelerates at a constant rate of \(a \mathrm {~m} \mathrm {~s} ^ { - 2 }\) until it reaches a speed of \(16 \mathrm {~ms} ^ { - 1 }\). It then travels at this constant speed before decelerating at a constant rate of \(0.75 a \mathrm {~m} \mathrm {~s} ^ { - 2 }\), coming to rest at \(B\). The total time for the journey is 240 s .

1. Sketch the velocity-time graph for the bus's journey from \(A\) to \(B\).
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2. Find an expression, in terms of \(a\), for the length of time that the bus is travelling with constant speed.
3. Given that the distance from \(A\) to \(B\) is 3000 m , find the value of \(a\).