A farmer has some chickens and ducks. If he sells 2 chickens and 3 ducks every day, there will be 50 chickens left when all the ducks have been sold. If he sells 3 chickens and 2 ducks every day, there will be 25 chickens left when all the ducks have been sold.

a) how many ducks are there?

b) how many chickens are there ?
类似的题还有一些，涉及到同一个总数的两种分法，每种里面用不同的unit。画图也麻烦。
用方程简单，就是二元一次方程组。