How many cows does it take to eat a field that keeps growing back?
Isaac Newton posed a puzzle in his algebra textbook: a pasture's grass grows at a constant rate every day, even as cows eat it down. Given how many cows it takes to clear one field size in a certain number of days, and a different number of cows for a different field and timespan, can you work out how many cows it would take to clear a third field in yet another number of days?
Reveal the answer
Yes -- by setting up equations that separate the grass already there from the grass still growing while the cows graze, the fixed daily growth rate can be solved for algebraically. Newton included this 'pasture problem' in his 1707 Arithmetica Universalis, and cattle-and-grass variants are still used today to teach simultaneous rate equations.