The pheasants and rabbits in a cage
A cage holds nothing but pheasants and rabbits. Counting heads from above gives 35. Counting feet from below gives 94. How many pheasants and how many rabbits are in the cage?
Reveal the answer
23 pheasants and 12 rabbits. Pheasants have 2 feet and rabbits 4. If all 35 animals were pheasants there'd be 70 feet, 24 short of the actual 94; swapping a pheasant for a rabbit adds 2 feet each time, so 24 divided by 2 gives 12 rabbits, leaving 23 pheasants. It's one of the oldest 'simultaneous equations by reasoning' puzzles on record, from the ancient Chinese text Sunzi Suanjing — the same text whose remainder problem gives the Chinese remainder theorem its name.