Hey Jorge,
This is the basic idea, I just copy/pasted/wrote really quick, so better check it.
When select is the multiselect and myCheckbox is the theckbox....
$(document).ready(function() {
$( "select" ).change(function () {
$( "select option:selected" ).each(function() {
/*do something here to make the checkbox logic and check the box using this: */
$('.myCheckbox').prop('checked', true);
});})});
Reference
1) :checkbox Selector | jQuery API Documentation
2) .change() | jQuery API Documentation<--- Check the demo at the bottom here
So the mapping would be, if they click on option A, checkbox 1 &2 get checked, if they click on option B, checkbox 1 & 3 get checked..... and so forth. The mapping would be manual.
Also you have to consider unchecking the boxes, which is also covered in ref 1.