



 
Nice question! Never take things for granted. To be specific, this what we do when a fraction or decimal expressing a portion of ONE unit is changed to a percent. It's not what we'd do if we were changing (say) 5g of water in 500g of margarine, or 0.05 liters of alcohol in 0.5 liters of wine, or three eggs in a dozen. Now, "percent" means "(parts) per hundred" ("centum" in Latin) so 50% means "fifty parts per hundred," etc. The percent sign is just that  a sign meaning "parts per hundred." So, to scale from "amount per 1 unit" to "amount per 100 units" we multiply by 100. And we add the percent sign to show we've done it. Good Hunting!  


Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences. 