"a" and a denominator "b". The value of a and b may be a real number or integer. The inverse of a ratio r is 1/r = b/a. A rate may be equivalently expressed as an inverse of its value if the ratio of its units is also inverse. For example, 5 miles (mi) per kilowatt-hour (kWh) corresponds to 1/5 kWh/mi (or 200 Wh/mi). Rates are relevant to many aspects of everyday life. For example: How fast are you driving? The speed of the car (often expressed in miles per hour) is a rate. What interest does your savings account pay you? The amount of interest paid per year is a rate. Rate of change Consider the case where the numerator of a rate is a function where happens to be the denominator of the rate . A rate of change of with respect to (where is incremented by ) can be formally defined in two ways: where f(x) is the function with respect to x over the interval from a to a+h. An instantaneous rate of change is equivalent to a derivative. For example, the average speed of a car can be calculated using the total distance traveled between two points, divided by the travel time. In contrast, the instantaneous velocity can be determined by viewing a speedometer. Temporal rates In chemistry and physics: Speed, the rate of change of position, or the change of position per unit of time Acceleration, the rate of change in speed, or the change in speed per unit of time Power, the rate of doing work, or the amount of energy transferred per unit time Frequency, the number of occurrences of a repeating event per unit of time Angular frequency and rotation speed, the number of turns per unit of time Reaction rate, the speed at which chemical reactions occur Volumetric flow rate, the volume of fluid which passes through a given surface per unit of time; e.g., cubic meters per second Counts-per-time rates Radioactive decay, the amount of radioactive material in which one nucleus decays per second, measured in becquerels In computing: Bit rate, the number of bits that are conveyed or processed by a computer per unit of time Symbol rate, the number of symbol changes (signaling events) made to the transmission medium per second Sampling rate, the number of samples (signal measurements) per second Miscellaneous definitions: Rate of reinforcement, number