Weird Remainder, Divisor, Quotient Rule from ETS

GRE Quant Problem Solving
1

Screen Shot 2021-10-15 at 2.32.38 PM
Screen Shot 2021-10-15 at 2.32.38 PM872×110 16.2 KB

Does this make sense to you guys, I kind of get it but its a bit weird to wrap ones head around