To calculate the average height of the five plants, you will need to sum their heights and then divide by the number of plants (which is 5).
The heights measured are: 15, 18, 21, 19, and 20 cm.
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First, add these values together: \( 15 + 18 + 21 + 19 + 20 = 93 , \text{cm} \)
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Next, divide the total by the number of plants: \( \frac{93}{5} = 18.6 , \text{cm} \)
Thus, the final average height for this group of plants is 18.6 cm.