Average height

There are twice as many girls in the class as there are boys. The average height of girls is 177 cm, boys 186 cm. What is the average height of students in this class?

Correct result:

a =  180 cm

Solution:

$$\begin{gathered} d=2c \\ a={\displaystyle \frac{177\cdot d+186\cdot c}{d+c}} \\ a={\displaystyle \frac{177\cdot 2c+186\cdot c}{2c+c}} \\ a={\displaystyle \frac{177\cdot 2+186\cdot 1}{2+1}}=180\text{ cm} \end{gathered}$$

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