Math tips, Math formulas

The fundamental concept to remember when performing calculations with percentages is that the percent symbol can be treated as being equivalent to the pure number constant 1 / 100 = 0.01. , for an example 35% of 300 can be written as (35 / 100) × 300 = 105.




To find the percentage that a single unit represents out of a whole of N units, divide 100% by N. For instance, if you have 1250 apples, and you want to find out what percentage of these 1250 apples a single apple represents, 100% / 1250 = (100 / 1250)% provides the answer of 0.08%. So, if you give away one apple, you have given away 0.08% of the apples you had. Then, if instead you give away 100 apples, you have given away 100 × 0.08% = 8% of your 1250 apples.



To calculate a percentage of a percentage, convert both percentages to fractions of 100, or to decimals, and multiply them. For example, 50% of 40% is:



(50 / 100) × (40 / 100) = 0.50 × 0.40 = 0.20 = 20 / 100 = 20%.

It is not correct to divide by 100 and use the percent sign at the same time. (E.g. 25% = 25 / 100 = 0.25, not 25% / 100, which actually is (25 / 100) / 100 = 0.0025.)



The easy way to calculate Addition in percentage (discount 10% + 5%):



y = [(x1+x2) - (x1*x2)/100]



for example: Dept Store promotion: discount 10%+5%, the total discount is not 15%, but:



y = [(10% + 5%) − (10% * 5%) / 100] = [15% − 0.5%] = 14.5%