Percentage calculation originated in ancient Rome in the first century BC. During this period, the Roman emperor created a decree declaring that various products sold would be taxed according to the amount traded for each type of product.

The term percentage comes from the Latin expression "*per centum*" and the calculations are carried out proportionally to the total amount, which in turn is 100%. For example, suppose it will be necessary to pay a percentage of 6% on the amount traded, this means that every 100 shares 6 must be the amount paid in fees. In other words, for every 100 oranges traded, 6 must be sent to the emperor as tribute and the rest, which will total 94, will be for sale!

This symbol is relatively recent, in ancient Rome Roman numerals were used followed by some letters such as: "IX pc" (nine percent) or "XIII per c" (eight percent).

One of the taxes created by the Romans was called "*centesima rerum venalium*" which corresponded to a hundredth part of the merchandise sold. The calculations were made without the percentage calculation that we know today and operations with centesimal fractions were used.

Currently fractional operations are widely used in stores, financial transactions, loans and in the sale and purchase of shares in the financial market.

Let's go to the calculations

Suppose you want to calculate 40% of $80. The whole represents the $80 and we want to know which part will be the 40%. We can represent 40% as a hundredths fraction, for example 40/100, and then multiply by the value that represents the total value. Then we will have:

(40/100)x80 = (40x80)/100 = (4x8x100)/100 = (4x8) = $32

Another example:

**Let's calculate 15% of $30. We will have**:

(15/100)x30 = (15x30)/100 = 450/100 = $4.5

We can think of a percentage as a part of the whole! This means that 10% is the tenth part of 100% which is better known as the total or $80 dollars! Let's use the previous example to calculate which part of $80 dollars corresponds to 40%.

- 10% ⇒ 1/10 of 100% ⇒ 1/10 of $80 ⇒ $8 dollars

- 40% ⇒ 4 times 1/10 of 100% ⇒ 4 times 1/10 of $80 ⇒ 4x$8= $32 dollars

This is really cool!! It means that we can easily calculate percentage using proportion. Let's try something more advanced.

**Calculate 13.5% of $80 dollars.**

- 10% ⇒ 1/10 of 100% ⇒ 1/10 of R$80 ⇒ $8 dollars

- 1% ⇒ 1/100 of 100% ⇒ 1/100 of R$80 ⇒ $0.8 cents

- 1% ⇒ 1/100 of 100% ⇒ 1/100 of R$80 ⇒ $0.8 cents

- 1% ⇒ 1/100 of 100% ⇒ 1/100 of R$80 ⇒ $0.8 cents

- 0,5% ⇒ half of 1/100 of 100% ⇒ half of 1/100 of $80 ⇒ $0.4 cents

- 13,5% ⇒(13.5/100) of 100% ⇒ (13.5/100) of $80 ⇒ 8+3x0.8+0.4 = $10.8

We can even use pencil and paper, but with a little practice we can do it in our head! Let's try another example.

**Calculate 21.75% of $40 dollars.**

- 10% ⇒ 1/10 of 100% ⇒ 1/10 of R$40 ⇒ R$4 dollars

- 10% ⇒ 1/10 of 100% ⇒ 1/10 of R$40 ⇒ R$4 dollars

- 1% ⇒ 1/100 of 100% ⇒ 1/100 of $40 ⇒ $0.4 cents

- 0.5%=>half of 1/100 of 100%=>half of 1/100 of $40=>$0.2 cents

- 0.25%=>quarter of 1/100 of 100%=>quarter of 1/100 of $40=>$0.1 cents

- 21.75%=>(21.75/100) of 100%=>(21.75/100) of $40=>2.4+0.4+0.2+0.1 = $8.7