An excellent resource, that I just happened to stumble upon today, is the Microsoft Math Worksheet Generator.  This is a FREE add on to Microsoft that enables teachers to quickly and easily make math worksheets complete with answer key.  The generator takes one sample question, inputted by the teacher, and generates other similar questions and an answer key.  

This add on program from Microsoft, once installed, places a shortcut on the desk top that allows users to access the program.  When you open the generator the first screen you see allows you to input a sample question of your own or choose one from the samples or recent problems.  The opening screen looks like this:

After inputting the sample question and indicating how many problems you would like the generator to create you click "Create" and you get a new worksheet complete with an answer sheet:

Worksheet and Answer Key:

Hopefully this program will benefit you in the classroom and be a good resource for creating practice math problems for your students.