The grass on a farm is growing at a constant rate. $$27$$ oxen can finish eating the grass on the farm in $$6$$ weeks. $$23$$ oxen can finish eating the grass on the farm in $$9$$ weeks. How many weeks it will take for $$21$$ oxen to finish eating the grass on the farm? 
Overseas Competition->Knowledge Point->Word Problem Modules->Newton's Problem of Cows and Fields->Basic Newton's Problem of Cows and Fields