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How to Work Out Percentages - Some Tricks to Help You Work Out Percentages Without A Calculator
If you're looking for conventional/standard ways to do percent problems you'll find several links to "real" math sites by scrolling down toward the end of the main part of this Hub. That isn't the purpose of this Hub; and although I've considered including more conventional approaches here, I think it's most fair to people looking for help with percent problems to direct them to sites that do a better job showing those conventional approaches than I can. After all, that's the sole purpose of those sites. For more on the purpose/intent of this Hub see "More About This Hub" below.
Some Handy "Tricks" for Doing "Percent Problems"
A Few Notes On The Basics:
In order to find x percent of any number the general way of doing that is to multiply the number by the percentage. Since percentage means hundredths of any number, percentages are written with the decimal point moved two places toward the left-hand side. With whole numbers the decimal point does not show, and since moving the decimal point over one place would indicate tenths, examples of how percentages should be written before multiplying are: .05 (five percent) or .20 (20 percent). An example is find 40% of 80: Multiply 80 (since its a whole number you don't see the decimal point) by 0.40 (which is how percentages can be written, although the 0 before the decimal point isn't really necessary). The answer is 32.
Multiplying any number by any percentage (and remembering to include the decimal points in the answer) will give the percentage.
Quick tricks for finding percentages can be used. For example, if one wants to find 30% of a number s/he can start with the original number, mentally move the decimal point over one place (in order to find 10% really quickly) and then multiply that 10% by 3 (because 10 x 3 =30). Any time the percentage involved is divisible by 10 this trick can be used.
Another quick trick for finding 5 percent is to move the decimal point over one place (again, to find 10 percent easily) and divide that result in half (because half of 10 is 5).
Finding percentages that are multiples of 5 (for example, 25) can be done in three steps: Move the decimal point over one place to get 10%, divide that figure in half to get 5%, and then mulitply that figure by 5 (because 5 x 5 = 25).
Also, knowing that 10% equals 1/10th of any number means that finding 10% can also be done by dividing by 10.
Most people are familiar with the fact that a quarter (25 cents) is 1/4th of 100 and that 50 cents is 1/2 of 100. Keeping those basics in mind, one can easy remember that finding 25% of any number means dividing it by 4 and that finding 50% of any number means dividing it by 2.
In the case of something like 75% figuring that out could be done either by using the above divisible-by-5 method or, if its easier, finding 25% and multiplying that by 3 (because 3 x 25 = 75).
A trick for finding 40% might be moving the decimal point over one to get 10% and either remembering that figure or writing it down. Then divide the number for which you're trying to find 40% of by 2. Once you have half of the number subtract that 10% you first figured out - and you've got your 40%.
The finding-10% trick/aid can also be turned into a finding=1% trick/aid. If you move the decimal point over two places you have found 1% of any number. You can easily multiply that 1% to get any other percentage simply by asking "how many 1's are in this percentage?". For example, to find 20 percent you can first find 1% and then multiply that 1% by the 20 (because there are 20 1's in 20). This will also let you find, for example, the 20%.
Finally, there is the issue of figuring out a tip in a restaurant. Again, the quick way (for 15%) is to move the decimal over one place to get the 10%, divide the 10% to figure out what 5% is, and then add together the 10% and 5%. A 20% tip is easier - just get the 10% and multiply it by 2.
Percentages - Links
A Few Primary-School Math Programs
More About This Hub
This Hub was originally written to show "quick mental tricks" for doing percent problems. It was written with adults in mind and in response to a request. It was not intended to offer more than those quick tricks for common percent problems. The aim was to offer people those tricks they could use in situations involving things like shopping and tipping. Sometimes we need a quick way to figure a tip or sales tax. Sometimes we want to know how much a product will cost when we learn it has a "75%-off price". The aim was to offer a few simple ways people can do percentage problems in their head that can come in handy, and that don't show up too frequently in the "standard" search on doing percentage problems.
When I wrote the Hub I didn't change the wording in the request, and now that it has taken on a nature/life of its own it is clear the title is no longer appropriate or sufficient (to describe the content). I could change the title, of course, but the Hub has found its place in search engines and has turned out to help a lot of people (usually young students) who were having some trouble "getting" how to do percentages. As a result, I don't want to change the title and risk the Hub's "getting lost to the ages".
The nature of the Hub evolved, though, because readers began asking for elaboration on the original approach (the "mental tricks") . From there, readers began asking about percent problems that went beyond the kind one would do using those "mental tricks".
Originally, I had approached writing this Hub (in answer to that question/request someone had posted) as if I were sitting at the dining room table, trying to help a middle-school or junior-high student understand percentage problems. So, it started out "folksy" and in terms that wouldn't show up in math books; and as things evolved and all those questions/comments continued to be posted, I continued to use the same approach.
The way I've seen it, if someone is asking how to figure a percent problem there's a good chance it's because they haven't already become comfortable "doing percents" as taught/being taught in school. So, as questions about finding percents have continued to come in (and as a lot of them have gone way beyond the original intent of this Hub), I've found myself using whatever words or combination of words and techniques I can "pull out of the air" in an attempt to make understanding what "is going on behind a percent problem". People who have trouble "getting" some things in math have that trouble because things like equations on paper don't mean anything to them.
In any case, the Hub has taken directions that weren't the original intent. At the same time, when new questions have been posted I've just figured, "It's easy enough for me to just answer them." As a result, the comments section of this Hub has become very much aimed at people for whom math class and math books haven't happened to be very effective (or wasn't effective enough for them to remember if they were students years ago).
The point is, if you're comfortable with math and having no particular challenge learning from math teachers or books, 1. You either don't need this Hub or else you need one of those conventional "how-to-do-math" sites, from which you can easily find that little extra bit of information you need., and 2. There's a good chance you'll find this particular absolutely "wacky".
If you've always been comfortable learning math there's also a good chance you can't imagine how a perfectly "smart" student may have trouble learning one thing or another, only because an adult hasn't figured out that he needs someone to think up a different way of presenting one thing or another.
Adults who may have, as students, once hit a "stumbling block" on one math thing or another, and adults who have ever had to watch a student struggle with one kind of problem or another (or most of them), will understand why I've left all the questions and answers in the "Comment" section on this Hub.