In this paper, we confirm through surveys and interviews that it helps students in solving a problem counting the number of combinations to find a structural isomorphism between the given problem and a typical problem with the same mathematical structure. Then we suggest that a problem of distributing balls into boxes might be a good candidate for a typical problem. This approach is coherent to the viewpoint given by English(2004) that it is educationally important to see the connection and relationship between problems with different context but with similar mathematical structure.