Explanation:
0 <= income <= 31000 : 10.8%
31000 < income <= 31000+36000 = 67000 : 12.75%
67000 < income < anything above 67000 : 17.4%
the total income is
$85,000
the taxes paid are
10.8% of 31000 = $3348
12.75% of 67000 - 31000 = 12.75% of 36000 = $4590
17.4% of 85000 - 67000 = 17.4% of 18000 = $3132
so, out of $85000 you paid
3348
4590
3132
----------
$11,070
taxes
100% = 85000
1% = 100%/100 = 85000/100 = 850
how many % are 11070 ?
well, as many as how often 1% fits into that amount :
11070 / 850 = 13.02352941...% ≈ 13.02%