Answer
Year $2,042$
Work Step by Step
Assuming that this trend holds for every year, we can model the described trend as follows: $$Tax_{rate} = 23 - 0.28y$$ where $y$ represents every year that passes after 1960. To find when the tax rate is zero, $$(0) = 23 - 0.28y$$ $$0.28y = 23$$ $$y = \frac{23}{0.28}$$ $$y \approx 82$$ which means that, should the trend have held true, corporate taxes would've reached zero in the year $1960 + 82 = 2,042$.
