Four persons can cross a bridge in 3,7,13,17 minutes.Only two can cross at a time.Find the minimum time taken by the four to cross the bridge ?

First send 17 and 7. After 7 minutes, 7 is at finish and 17 needs 10 more minutes to complete his travel. now send 13. After 10 minutes 17 reaches finish and 13 needs 3 more minutes to complete. now send 3. both 3 and 13 reach destination at the same time after 3 minutes. hence the time taken is 7+10+3=20 minutes.