A fifteen feet long ladder is placed across a street such that while its base is at one edge of the street its top rests against the opposite wall at a height of nine feet. Similarly, another ladder, twenty feet long, is placed resting across the other side-wall so that the two ladders cross each other. A spider wishing to cross the street, climbs up one ladder till it gets to the meeting point; thereafter, it climbs down the other.

How long will the spider take to accomplish the crossing assuming that he covers a foot in ten seconds?