Twelve oxen were put to graze in a field of 3⅓ acres. After four weeks they had eaten all the grass in the field, including the grass which grew during the four weeks. At the same time 21 oxen were put out to grass in a field of 10 acres; they grazed it bare in 9 weeks. How many oxen would a field of 24 acres support for 18 weeks, if they were put in it at the same time? (See Isaac Newton’s Arithmetica Universalis.