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.