A stereo that normally sells for $125 is on sale for $90. By what percent has the price of the stereo been reduced?

28%

so you subtract $90 from $125 and that gives you $35, then you must determine the percentage by trial to see what percent that $35 is of $125, you can tell by looking that it will be near to a third or 33%, but not quite that much, so work backwards from there and try 30% and because that is just a bit more that $35, reduce and try a smaller percentage, 28% of 125 is exactly $35

The answer is approximately 28%