percent 'of'

a ppt is here

it's a historical quirk that we have percentages
but they appear to have been around for a while

Wikipedia claims that they were evident in Ancient Rome,
emperor Augustus levied a tax of 1/100 on goods sold at auction

as denominations of money grew in the Middle Ages, computations with a denominator of 100 became more standard and from the late 15th century it became common for arithmetic texts to include such computations
many of these texts applied these methods to profit and loss and by the 17th century it was standard to quote interest rates in hundredths

as '% of' operators, maybe it is simplest if percentages are turned into decimals from the outset, and ongoingly...

see who can get nearest

to be done mainly mentally, with jottings

questions quite often relate to another one