As a decimal, 9% is 0.09, because the symbol % means percent, which in turn means literally "per one hundred". 9%, or in words, nine percent, means nine per hundred. For example if 9% of people prefer tea to coffee, that means that out of every hundred people, nine prefer tea to coffee. We use percentages because they are a convenient way of expressing proportions and ratios. If one wants to convey that the price of some item has been halved, they might say that there is "50% off" because when dividing the total cost of the product into one hundred sub-units, the price reduction is the equvalent of discarding half of these units. Percentages are also useful in situations where using fractions to display proportion are clumsy, and the decimal point in a decimal number is easily overlooked. For example, it is neater to say that 0.068% of a sample of air was carbon dioxide rather than saying that seventeen two hundred and fiftieths (17/250) or 0.00068 of the air was carbon dioxide. To convert a percentage to a decimal all you have to do is divide by one hundred, which is equilalent to moving the decimal point (if there is one) two places to the left. Conversely to convert a decimal to a percentage you have to multiply by a hundred, which is the same as moving the decimal point to places to the right. A decimal is the extension of the base ten number system to encorperate fractions. From right to left, the places values increase in powers of ten; for example in the number 90, the fact that 9 is the second number reading from right to left means that there are nine lots of ten to the power one, or nine lots of ten, so ninety. The decimal point acts as a reference point from which powers of ten can be extended to cover the negative powers of ten. For example, in the number 120.034 the decimal point shows that the place value of 100 (the units) are one space to the left, then to the left of that there are 2 lots of 101, or twenty, et cetera. To the right of the decimal point, the 0 shows there are zero lots of 10-1, then to the right of that the 3 denotes three lots of 10-2, that is, three hundredths. Because we are in base, multiplying by any interger power of ten is extremely easy, all you have to do is put zeroes on the start of a decimal or end of the number as appropriate. For this reason, converting between percentages and decimals is extremely simple.
