Velocity from position–time relation

2016 Phase II 1 question

1. 1 Motion in a Straight Line
   Two cars P and Q start from a point at the same time in a straight line and their positions are represented by $x_P(t) = at + bt^2$ and $x_Q(t) = ft - t^2$. At what time do the cars have the same velocity?
   A $\frac{a + f}{2(1 + b)}$ B $\frac{f - a}{2(1 + b)}$ C $\frac{a - f}{1 + b}$ D $\frac{a + f}{2(b - 1)}$
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