So ive got a drop down select menu with options 1 - 10. (i.e. "1" "2" "3" etc..)

When the user selects 3 from the select box, i need to do a math equation of 3 x 10 and put the result of 30 into a text field.

Anyone got a function that might help?

Something like this would work:

function selectMultiply(){
  var objSelect = document.getElementById('id_of_select_element');
  var objText   = document.getElementById('id_of_text_input_element');

  objText.value = objSelect.value * 30;
}

On your select element, add the attribute onchange="selectMultiply()".

w3schools has a fairly decent reference to the HTML DOM for future reference.