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For 1 additional tree: Yield = (300 -1(10)) (20 + 1)

For 2: Yield = (300 -2(10)) (20 + 2)

What's different between those two? Only the values of 1 or 2. So for an unknown number of trees, replace the 1 or 2 with an X to get your formula:

Then transpose that to get:

Note that 250 was used merely to simplify the formula. The author skipped a step, which is causing your confusion.

They started with this:

And then both added and subtracted 250, for a net change of zero:

And then simplified that into: