Think about what you've been actually doing when you calculate the average of two numbers, let's say 80 and 60. If you add them and divide by 2, you get 70. What you're really doing is saying that each number contributes 50% (or 0.5) of the total:
80 x 0.5 = 40
60 x 0.5 = 30
40 + 30 = 70!
So in your problem, you just need to multiply each number by its worth and add up the products.