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Quantity or amount is how much of something there is that can be counted or measured.
Whole numbers (1, 2, 3 ...) are used to count things. This can be done by pointing to each one. As things are pointed to, a number is said. Start with the number one. Each time another thing is pointed to, the next whole number is used. When the last thing in a group it pointed to, that number is the quantity of the group.
Measuring is done with a ruler or a machine. We put a ruler next to a thing to measure how long it is. We put things on a scale to measure their mass. Other machines are used to measure other quantities such as temperature, speed, electric current, and so on.
When measuring things, a whole number might not be the best answer. A distance might be longer than 5 meters, but less than 6 meters. Meter sticks are marked off in parts of a meter. Decimals are formed by marking off a ruler in ten equal parts. Each part is a tenth. 5.2 meters is a little bit longer than 5 meters. 5.7 meters is a little bit shorter than 6 meters. Each of these tenths can be marked with ten smaller parts. The one hundred centimeters on a meter stick are done this way; by marking ten equal parts, then marking ten equal parts of each of those parts.
Fractions are used when there are parts of a whole number. The dial on a machine may be marked with four parts between each whole number. A scale with a bag of potatoes may show five and three-quarters kilograms. This would be almost six kilograms.
The words "amount" and "number"
People often use the word "amount" when they should say "number". The words "number" and "amount" should be used if a particular number could be put: e.g.
- A large amount of sand (because you cannot say "three thousand sands")
- A large number of people (because you could say there were 3547 people).