I can't find any information on how to do this other than through some sort of goal-seek. Ideally I'd like a formula-based solution, if one exists.

Basically I need to 'reverse-engineer' the NPV calculation to find the rate. I've illustrated my problem in the image below (I'm trying to find the value in the yellow-highlighted cell).

I have two ways of selling a product with equal annual cashflows:

- Option A assumes a cost of capital of 10% and calculates the five even annual cashflows required to reach a target NPV (£20k).
- Option B starts with a given figure for total cashflows (£30k in this example) and divides that by five to get equal annual cashflows.
**I want to know the effective cost of capital / interest rate applied to make the NPV of this option the same as option A (£20k).**Is this possible?

Thanks for any advice.