Formula to Calculate Percent IncreaseDate: 01/31/2005 at 16:58:20 From: Rachel Subject: How do I calculatre percentage increases Hi, I want to compare two figures and show the percent increase that has been made. For example, if 55 has increased to 75, what percentage increase is that? Whenever I get a figure, I am never sure if there is a way of checking back that it is correct. Date: 02/01/2005 at 16:05:06 From: Doctor Ian Subject: Re: How do I calculatre percentage increases Hi Rachel, When we say Q increased by P percent what we _mean_ is that the new value of Q is Q' = Q + (P percent of Q) which is to say, Q' = Q + (P/100 * Q) So far, so good? If you have Q (the old value) and Q' (the new value), then you want to find P. If we solve the equation for P, we get Q' = Q + (P/100 * Q) Q' - Q = P/100 * Q Q' - Q ------ = P/100 Q Q' - Q ------ * 100 = P Q So you subtract the old value from the new one, and divide the difference by the old value. Then multiply by 100 to get the percentage. Another way to think of that is: amount of change ---------------- x 100 = percent change original amount If you want to check your work, you can go back to the definition, plug the numbers in, and see if what you get is true. For example, suppose I have an increase from 40 to 50. The formula gives me 50 - 40 10 P = ------- * 100 = -- * 100 = 25 40 40 So I think it's 25%. Let's go back to the definition: 50 = 40 + (25% of 40) = 40 + 10 = 50 This is true, so I have the right percentage increase. Does this make sense? - Doctor Ian, The Math Forum http://mathforum.org/dr.math/ |
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