MATH PUZZLE: Can you replace...

Groups of Americans were traveling by tour bus through Holland. As they stopped at a cheese farm, a young guide led them through the process of cheese making, explaining that goat's milk was used. She showed the group a lovely hillside where many goats were grazing.
"These," she explained, "are the older goats put out to pasture when they no longer produce." She then asked, "What do you do in America with your old goats?"
A spry old gentleman answered, "They send us on bus tours!"