Rates and ratios often vary with time, location, particular element (or subset) of a set of objects, etc. Thus they are often mathematical functions.
A rate (or ratio) may often be thought of as an output-input ratio, See more

• Birth rate, and mortality rate, the number of births or deaths scaled to the size of that population, per unit of time
• Literacy rate, the proportion of the population over age fifteen that can read and write
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In chemistry and physics:
• Speed, the rate of change of position, or the change of position per unit of time
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• Exchange rate, how much one currency is worth in terms of the other
• Inflation rate, the ratio of the change in the general price level during a year to the starting price level See more

• Derivative
• Gradient
• Hertz
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