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Four people are in need to cross a river at night time.
each one of them can cross the river at a differet speed :
Person A can cross the river in 1 hour
Person B can cross the river in 2 hours
Person C can cross the river in 4 hours
Person D can cross the river in 5 hours
the calculated time for crossing is according to the slowest person
In order to cross the river , a flashlight can be used but only 2 persons
can use it at a time. the flashlight is good for 12 hours.
how would all of them will cross the river?
They have a flashlight