JetMouse
My thoughts on this are as follows:
Interpolation is based on two known factors to find a third.
WIRELESS is correct in his working but I am not sure you could call it interpolation. We were given 2000ft and 1% as the only data and asked to find a third value by interpolation. This is not possible with the given data so we must make some assumptions.
I will assume 2000ft and 1% are the LOWER values. I will also assume that the interpolation is linnier so I can assume the UPPER values to be 4000ft and 2%. NOW I have enough data to Interpolate any value between 2000ft and 4000ft.
Given 2100ft I came up with 1.05% which is correct. 2350ft is 1.18% (2dp)
I made a very handy Interpolation formula in excel that I would be happy to email to you...