Need some array help

So I have Array A and Array B; I have to multiply them together (Array C), sum Array C, and then divide Array C by the sum of Array B

Example:

A - B
0.5 - 150
0.4 - 200
0.2 - 400

So what I usually do is: sum((A2:A4)*(B2:B4))/sum(B2:B4) ctrl+shift+enter, but I want to create a function that does this for me where I could just enter the range A2:B4 as the input argument

Any help? I have been trying but Excel is being a pain with the summing of Array B