MySQL: adding and multiplying on different tables

I have two tables, one contains data like this:

link_id | coun开发者_JAVA技巧ted
=================
1--------| 1
==================
2------- | 0
==================
3 -------| 1
===================

I want to select those that are counted = 1, and then with the ids here I want to go to the table link (whose ids are in the link_id table above), and multiply each by its corresponding factor:

id | factor
===========
1  |  0.3
============
2  |  0.1
===========
3  |  0.5
==========

So for the values above it would be:

counted = 1 in first table, 1 and 3. Now,

1*0.3 + 3*.5 = 0.3+1.5 = 1.8

How can I do this with a MySQL query?

---

SELECT SUM(first_table.link_id * second_table.factor) as ANSWER
FROM   first_table
LEFT JOIN second_table on first_table.link_id = second_table.id
WHERE  first_table.counted = 1