You might want to look at my previous post which explains what fractions mean.
PercentagesAs you should know, if for example you have a 10% income tax, that means that out of every 100 units of income you make, 10 units of them are taxes. So a percentage means the number of units you need to take out of every 100 units of a total. This is why it's called "percent", that is, "per hundred", because you are finding the amount of units you need to take our of every hundred units in the total.
However we can also express this statement using fractions instead of percentages. We can just say that 1/10 of your income is taxes. In fact we can convert fractions into percentages by multiplying the fraction by 100, 1/10 × 100 = 10, that is 10%.
Before we understand percentages, we need to understand fractions of totals.
The statement A/B of C means two things:
- As explained in the last post, it means divide C into B equal parts and take A such parts.
- It also means A is the number of units to take out of every B units in C.
We can either break 20 into 4 equal parts and take 3 of them:
20 = 5 + 5 + 5 + 5 (4 equal parts)
take 3 of the parts and we have
5 + 5 + 5 = 15
Or we can take 3 from every 4 in 20:
20 = 4 + 4 + 4 + 4 + 4
take 3 from every 4 and we have
3 + 3 + 3 + 3 + 3 = 15
In general, if we want to take A/B of C,
C = C/B + C/B + C/B ... (for B times) (B equal parts, that is, C/B × B which is equal to C)
take A of the parts and we have
C/B + C/B ... (for A times) = C/B × A
Or we can take A from every B in C,
C = B + B + B ... (for C/B times) (that is, B × C/B which is equal to C)
take A from every B and we have
A + A + A ... (for C/B times) = A × C/B
Since C/B × A = A × C/B, we know that the two statements are equal.
Good. So now we return to percentages. The reason why we convert fractions to percentages by multiplying the fraction by 100 is the following:
Given a fraction A/B, when we convert it to a percentage, we are changing the denominator of said fraction to 100 but leaving the fraction equal to A/B, and taking the numerator. So A/B becomes P/100 and P is the percentage.
We are finding a number which when divided by 100 gives the original fraction and therefore the amount of units you need to take from every 100 units of a total such that when you divide the amount you took by the total, you get the original fraction.
For example, if you have a total of 50 units and you want to take 1/10 of the total, the number of units you must take from the total, when divided by 50 must result in 1/10. Likewise, if we change the denominator of the fraction to 100, that is, 10/100, then we say that 10% of 50 units is the number of units we must take such that when it is divided by 50 we get 10/100 (which is equal to 1/10).
So what we're doing is finding another fraction which is of the form P/100. However, P/100 must equal A/B in order to remain the same fraction.
So we have the equation A/B = P/100
We want to find P, so P = A/B × 100 (multiplied both sides by 100)
So if we want to express 2/4 as a percentage,
2/4 = P/100
P = 2/4 × 100
P = 50
So 2/4 = 50/100 or 50%.
And this is why percentages work this way.
In generalNow we can generalize this to numbers other than 100. If we use "X" instead of "100",
A/B = P/X
P = A/B × X
By changing the denominator to X, the numerator P will be the amount of units you need to take out of every X units of a total, such that when you divide the amount you took by the total, you get A/B.
So, since P/X is equal to A/B, then just like we can say that P/X means that P is P/X of X, we can also say that P is A/B of X. For example, if 2.5/5 = 1/2, then just like 2.5 is 2.5/5 of 5, 2.5 is also 1/2 of 5.