I see so many educated people not realising this. The maths involved is something we learnt in ~ 5th grade, and I distinctly remember doing exercises on marginal rates in primary school in maths class. It's even simpler than compound interest - which is a staple of maths class later on.
Yet so many people say there's a problem with the education system that it doesn't teach practical skills like these. It clearly does, kids just don't remember it. Maybe it's because they don't need to use this knowledge until almost a decade later.
The US was still fighting in Vietnam while that was happening.