Answer
1950 to 1960,
91%.
Work Step by Step
During this period, the ten-year period during which the top marginal tax rate was constant is the period during which graph is exactly horizontal.
This happens between 1950 and 1960. So, the answer is 1950 to 1960.
Also, during this period the tax rate was slightly higher than 90%, so it can be estimated to be 91%.
So, 91% is the answer.
