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 ?  Choose the correct choice from the following:
A) 20 minutes.  B) 25 minutes.  C) 30 minutes.  D) 40 minutes.  E) 60 minutes.
Explanation:   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= 
<gadget id="calculator"> 7+10+3</gadget>
<output>20</output>
20 minutes.   20 minutes.nswer 20 minutes.
<result>A</result>
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 ?