Vector angle in multiple planes

How would I go about solving a problem that wanted you to find the angle between two vectors of equal distance given the length of these two vectors and the sum of the two vectors (let's say #j).

You can throw out any number for distance and the sum of the two vectors. I want to see how this is done is all.

edit:

I'll throw in some numbers if that helps. These aren't my actual numbers, but maybe it'd be easier to explain.

Two vectors of equal magnitude 60.

The sum of the two vectors is 20j.

What is the angle between them?