The effective marginal tax rate when a person moves from the bottom to the middle quintile is 76 percent

See An Effective Marginal Tax Rate by Greg Mankiw. 

"Interesting numbers from Phil Gramm and Robert B. Ekelund Jr.:

The bottom quintile earned 2.2% of all earned income in 2013, but after adjusting for taxes and transfer payments, its share of spendable income rose to 12.9%—six times its proportion of earnings. The second quintile’s share more than doubled, rising from 7% of earned income to 13.9% of spendable income. For the third quintile, middle-income Americans, the increase was much smaller, from 12.6% to 15.4%.

To put it another way, the effective marginal tax rate when a person moves from the bottom to the middle quintile is 1 - (15.4-12.9)/(12.6-2.2), or 76 percent."