The Lost Expedition

The Story: A team of four researchers — Dr. Nalini, Dr. Farhan, Dr. Keiko, and Dr. Torres — are returning from a remote field site in the Western Ghats when their jeep breaks down on a narrow mountain road at dusk. The nearest village is 10 kilometres away, but between them and the village is a rope bridge over a deep ravine. The bridge is old and can safely hold at most two people at a time. Night has fallen completely. They have one torch between them, and the bridge is too dangerous to cross without it. Nobody can cross without carrying the torch or being accompanied by someone who has it. Dr. Nalini, the youngest, can cross in 1 minute. Dr. Farhan needs 2 minutes. Dr. Keiko, who has a knee injury, needs 7 minutes. Dr. Torres, the eldest, needs 10 minutes. When two people cross together, they move at the pace of the slower person. The torch must be walked back after each crossing — it cannot be thrown. Nalini says, 'We must reach the village within 20 minutes or we will miss the last truck home.' Farhan takes out a notepad and starts calculating. The Challenge: Can all four researchers cross the bridge in 20 minutes or less? If yes, what is the minimum time required and what sequence of crossings achieves it?