I am not entirely sure that this question belongs in here, however most mental maths solutions regarding Relative Velocity involve trigonometry and so this is where I have posted it.
I think it is easier for me to post a problem that is similar, but to which I know the solution, before posing my questing to give some context.
eg. Two ships are travelling directly north at 12 knots. A bearing taken from one ship to the other reads 030. If either ship wants to open or close range on a steady bearing, then they need to steer left/right 6 degrees for every knot of speed advantage that they may have.
That is the simple mental maths solution for the problem, and I believe it is based on the Sine Rule.
My question is the opposite of this. Is there a mental maths solution to determine the course that would be required to come to, given that they have a specific speed advantage available and want to move a particular number of degrees around the circle, ending up at the same range from the ship?
Sorry this is a poorly worded question. I tried to create a drawing which better described it but was unable to do this computer.
Thanks for any help you can give.