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For things that grow, whether in height or weight, you can measure the rate of growth by finding the change in whatever quality you wish to measure, divided by the time. This formula can be expressed mathematically as:   rate=ΔhΔt{\displaystyle {\text{rate}}={\frac {\Delta h}{\Delta t}}} or ΔwΔt{\displaystyle {\frac {\Delta w}{\Delta t}}}  In these two examples, h{\displaystyle h} represents the height and w{\displaystyle w} represents weight. In both of these, t{\displaystyle t} is for the elapsed time. Some plants, like the Asian bamboo, grow very fast, with visible differences taking place within hours. For measuring something like the growth rate of a child, changes may not take place for months or a year or more. You need to select a time period that is relevant for what you are measuring.  Suppose an elementary class plants bean seeds and begins measuring their growth as soon as the first sprout appears. A reasonable time measurement might be about a month, measured in days. Scientists raising an orphaned baby elephant wanted to measure its growth rate over the first 90 days of its life. Measuring a growth rate requires that you set some starting point and measure the size at that time.  For the example of the students' bean plants, they chose the starting point to be the day that the first sprout appeared. The height at the point is set at 0 cm. For the baby elephant, veterinarians measured the elephant's weight the day it was born. Its initial weight on that day was 200 pounds. After the time elapses for your study, measure the height or weight of the object whose growth you are studying.  For the bean plants, the average height of the students' plants on the 30th day was 24 inches tall. Because the plants began at a height of 0, the amount of growth was 24 inches. For the elephant, after the 90 day study period, the veterinarians measured its weight to be 400 pounds. Enter the data that you measured into the formula and perform the calculations to find the growth rate.  For the students' bean example, the calculation will look like this: rate=24 inches30 days=0.8 inches day{\displaystyle {\text{rate}}={\frac {24{\text{ inches}}}{30{\text{ days}}}}=0.8{\frac {\text{ inches}}{\text{ day}}}}  For the elephant's growth rate, you must calculate the amount of change in the weight in the numerator as part of the calculation:  rate=400−200 pounds90 days{\displaystyle {\text{rate}}={\frac {400-200{\text{ pounds}}}{90{\text{ days}}}}} rate=200 pounds90 days{\displaystyle {\text{rate}}={\frac {200{\text{ pounds}}}{90{\text{ days}}}}} rate=2.22poundsday{\displaystyle {\text{rate}}=2.22{\frac {\text{pounds}}{\text{day}}}}
Understand the formula for measuring average growth rates. Decide how long you wish to measure the growth rate. Calculate the starting size. Measure the ending height or weight. Use the growth rate formula for either height or weight.