Two ships sail on the sea with constant speeds and fixed directions. It is known that at the distance between them was miles; at , miles; and at , miles. At what moment were the ships the smallest distance from each other, and what was that distance ?
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Two ships sail on the sea with constant speeds and fixed directions. It is known that at $9:00$ the distance between them was $20$ miles; at $9:35$, $15$ miles; and at $9:55$, $13$ miles. At what moment were the ships the smallest distance from each other, and what was that distance ?
Izvor: Međunarodna matematička olimpijada, shortlist 1968
Školjka 
the distance between them was 
miles; at 
, 
miles; and at 
, 
miles. At what moment were the ships the smallest distance from each other, and what was that distance ?