How Far

A fellow was walking along a country road and came upon a farmer working in his field.
The man called out to the farmer and asked how long it would take him to get to the next town.
The farmer didn't answer.
So, after waiting a bit, the fellow started walking again.
After the man had gone about 100 yards the farmer yelled, "About 20 minutes."
Confused, the man turned back toward the farmer and inquired,
"Why didn't you tell me that when I asked you?"
"Well," said the farmer, "I didn't know how fast you could walk."