ft - oh, indeed, elegance and simplicity, good sir - salut ! I shall away and beat myself into a pulp for not seeing it earlier.
However, lest a couple of unintended (I'm sure) typos in your post confuse our colleague, Pugachev Cobra, I suspect that you intended, for your solution
4200****20****84000
*500****30****15000
4700****21.1***99000
(or, 21.06 if one were to desire two decimals)
I presume that you did the calc and then retyped it with the first typo resulting in the second but the answer transcribed correctly from your original calculation ? However, 85500/4700 just doesn't quite fit the bill.
No matter .. I am more than impressed by your insight in the technique used .. well done, good sir.