Help with creating an index..!! urgent for student report.

We made an index, and need to change the variables so it goes from 0-10 instead of 3-15. How do we do this? we currently use the formula: ((lvnds1+pris1+Skatt1 ) - 3) / 1 but then we get an index that goes from 0-12. Anyone knows how we can get it to 0-10 instead?