a percent is simply a fraction or decimal represented as a part out of 100. since 50 is 12 of 100, the percent equivalent of 12 is 50%. it is also equivalent to 50100, which we know is .5. often in economics, we will want to say by what percentage something increases or decreases. for example, costs increased by 40% this month. or, gdp is down 2% this quarter. to calculate a percentage change, find the difference of two numbers (the change) and divide it by the starting point to get the percentage change. try this one: costs went from $80 last month to $100 this month. what was the percentage change in costs? input your answer in percent form.