A traveller walking along a road

A traveller walking along a road asked an old man working in a field to how long it would take to get to the next village. But the old man didn?t answer, so the traveller kept walking. He hadn?t gone far when he heard a call: ?Hi, mister, it?ll take you about 20 minutes?. ?Why didn?t you tell me that when I asked you?? asked the traveller. ?How did I know how fast you were going to walk?? replied the old man.