Moving The Curser From Text Box To Text Box Automatically When A Condition Is Met?

I have learned a good bit of PHP and have implemented it for use in my work as a math tutor.My kids are telling me, however, that it is too inconvenient to hit tab or click on the next text box and would prefer to use the old style pencil and paper.I thought of a good idea: Javascript would be able to automatically focus the curser on the next text box if some condition was met.For example: 6+7. If the textbox reads 13, it will focus on the next text box. Otherwise, nothing happens. This kills three birds with one stone; the user will know if they got the question right or wrong and it will move automatically if they got it correct.The problem is I lack any real JavaScript wisdom. I would guess this would be quite simple. The closest thing I have found upon searching was this from the user requestcode, but this has to do with once the user has typed in 4 characters it moves automatically.

<SCRIPT LANGUAGE="JavaScript">
function nextbox(fldobj,nbox)
{

[code]....