Logic puzzle 3

Level pending

Four people need to cross a dark river at night. They need torch to cross the river and at a time only 2 people at maximum can walk on bridge. They have only one torch. Speed of each person crossing the river is different. It is 3 mins, 4 mins, 10 mins and 20 mins respectively. What is the shortest time needed for all four of them to cross the river?

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