Hi, The aim of this store is to provide renewable resources that can be used over and over again without the hassle of having to find new problems. A teachers time is the most precious commodity and if these worksheets can save time, then they will have served their purpose. Think of the time I can save you. Nearly all of the resources I have are in an.xls format (Microsoft Exel). By pressing the F9 key, your worksheets will give you a new set of problems to work with.