$P$ and $Q$ travels from $D$ to $A$ and break journey at $M$ in between. Somewhere between $D$ and $M$, $P$ asks "how far have we travelled?" $Q$ replies, "Half as far as the distance from here to $M$". Somewhere between $M$ and $A$, exactly 300 km from the point where $P$ asks the first question, "How far have we to go?" $Q$ replies, "Half as far as the distance from $M$ to here". The distance between $D$ and $A$ is:
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