$55.85 is what percent of $351.00
translate into math:
$55.85 = x% * $351.00
x% = $55.85 / $351.00
x% = 0.1591 (roughly; exact number goes on forever)
"percent" sort of means "divide by 100," so:
x / 100 = 0.1591
x = 15.91 (roughly)
$55.85 is (roughly) 15.91% of $351.00