You are the onwer of a bird store, and you use them to organize your cages. If you keep 1 bird per cage, there will be one bird outside of the cage, therefore it will be missing cages. If you keep two birds per cage, there will be one empty cage. How many birds and cages do you have?
If you have three cages and four birds and you put one in each cage you have one bird outside but if you put two birds in each cage you have one cage left over.