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A comprehensive overview of various concepts related to percentages, including their basic uses, appropriate and common abuses, as well as formulas for calculating absolute and relative changes, comparisons, and compound interest. It delves into the nuances of percentage calculations, highlighting the importance of maintaining a consistent reference value and the potential pitfalls of averaging percentages. Additionally, the document covers key financial concepts such as installment loans, credit cards, mortgages, and prepayment strategies. The detailed explanations and examples make this document a valuable resource for understanding the practical applications of percentages and their role in personal finance.
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Definition of percentage - Per 100, how many parts per 100 Hundredths - 5% 5/100 or .05 are all the same in hundredths Tenths - 3/10 or .3 or 30% notice the decimal in in the tenth place 5% of a class of 40 students get an A equals how many students? - 5/100 * 40/1= 200/100 or 2 (students) What if you have a % in your decimal like .003%? - Take the number & write it over 100 to make a fraction- .003/ The Three basic uses of percentages -
Absolute Change Describing a Change in a value - describes the actual increase or decrease from a reference value to a (new) smaller or larger value. Absolute change=NV - RV new value minus the reference value (reference value is always the previous value, or, the New value minus the old, or new value-reference value Absolute Change for Comparisons of two different vaues - the actual difference between the compared value and the reference value. Absolute difference = compared value - reference value. AD=CV-RV Relative Change that Describes a Change - is a fraction that describes the size of the absolute change in comparison to the reference value: Relative change = the absolute change divided by the reference value which = the new value minus the reference value divided by the reference value. Relative Difference for Comparisons - describes the size of the absolute difference as a fraction of the reference value. Relative difference = Absolute difference divided by the reference value which is the compared value minus the reference value divided by the reference value. (pg 132) When a quantity doubles in value it's relative change is - 1 (RV) or 100% extra; the 100% is the difference between them so it's (RV or 1)+100% more When a quantity triples in value, its relative change is - 2 or 200% extra or loss if it's negative When a quantity quadruples in value, its relative change is - 3 or 300% extra or loss if it's negative. If $250 is increased by 500% you would multiply $250 by 500% + the original $250 because it 'Increased' The Relative difference formula gives a fraction which you can convert to a percentage by - Multiplying it by 100%
Translating a more than statement into an of statement - Retail prices are 100% + 25% = 125% of wholesale prices. You replace the of with multiplication. The retail price =125% times the wholesale price. If the compared value is P% more than the reference value then - the compared value = 100% + P% times the reference value and the reference value = the compared value divided by 100% +P% If the compared value is less than the reference value then - Use 100-P instead of + (plus) Shifting Reference Values - The reference value shifts during the problem; it can be higher or lower in the first calculation versus the second. Consider this statement, Your investment lost 60% the first year but gained 75% the second, so you're 15% ahead. An investment of $1,000 that lost 60% of its value would leave you with $400, in the second year your investment gained 75%, of $400, which would be $300, for a total of $700. This is less than your original investment not a gain of 15% overall. Don't average percentages; why? - Unless both tests or averages have the exact same numbers they wouldn't be an accurate average. You should NEVER average percentages. If you got 70% on one test and 90% on the second would your average be 80%? - Not unless they both had the exact same number of questions. If one has 10 questions & you got 7 correct, and the other has 100 & you got 90 correct, the combined total would be 97 correct out of 110 which is 88.2% because 97 divided by 110 is 0.882. pg 139 New or Changed Value equals - Reference Value times the percentage of Compared Value equals - Reference Value times the percentage of Simpson's Paradox -
It is possible for a set of data to give different results in each of several groups than it does when the groups are taken together. Overall results are divided into unequally sized groups. Check to see if groupings are unequal, look at total number of information. Numbers can deceive unless they are examined with great care. Numbers may not lie, but they may be deceiving when interpreted improperly. Examples of Simpson's Paradox -
Accumulated balance after Y years, also known as future value, or, FV P = - Starting Principal also known as present value, or, PV APR = - Annual Percentage Rate - must be written as a decimal in the formula. When interest is compounded just once a year. Y = - Number of years n = - Number of times it compounds; Bi-annually (every two years like bi-weekly is every two weeks), annually, Semi-annually (half a year), quarterly, monthly or daily. APY - Annual percentage Yield. The actual percentage by which a balance increases in 1 year. It's equal to the APR if interest is compounded annually & does NOT depend on the starting principal. Also known as the effective yield or just the yeild. (If no value is given just plug one in to find it) Compound Interest formula for more than one time a year - A= P (1+APR/n) ex (n*Y) Installment Loan - A loan you pay off with equal regular payments. Also called an Amortized loan. The portions of installment loan payments going toward principal and toward interest vary as the loan is paid down. Early in the loan most of it goes towards interest & gradually decreases as the portion toward the principal gradually increases. Installment Loan formula - PMT= P * (APR/n)
[1-(1+APR/n) ex (-nY)] To find payments on an installment loan -