Perhaps, we should give a little 'theory of knowledge' (ToK). In English, we ask: "if 4 farmers could finish harvesting a rice paddy in 25 days, how many days would it take 10 farmers to harvest the same paddy?" And ธิดาน้อย gave an answer (above) without giving her theory of knowledge. The theory is a simple one based on a concept of "work". Because 4 farmers could harvest the rice paddy in 25 days, the amount of work is simply "4 farmers times 25 days" giving "100 farmers-days" of "work". If this work is divided equally among 10 farmers, then each farmer would get "100 farmers-days divided by 10 farmers" or "10 days" after the units "farmers" are taken out in division. The matter is trivial but quite important in learning - we should be able to explain our answer and verify its correctness not just in mathematics but also in "theory" of reasoning ;-).