How to do some calculations using fields within a Field Collection field? I have a Field Collection field and I'm trying to do the following:

Multiply the value in one field with the value in another field and have the result shown within a third field.

I have tried Mathfield, but it doesn't support Collection Fields.

Is it possible to do this with Computed Field? How? Or would be difficult to write a custom module to do this?
You can do it by using Computed Field.
The sentence 
$sum = 0;
$wrapper = entity_metadata_wrapper($entity_type, $entity);
foreach($wrapper->field_FIELDCOLLECTION as $collection) {
$sum += $collection->field_SUBFIELD->value();
}
$entity_field[0]['value'] = $sum;