Three numbers

In three numbers, the first number is double the second number, and the second number is double the third. The average of the numbers is 21. Find the least number.

Final Answer:

c =  9

Step-by-step explanation:

$$\begin{gathered} a=2b \\ b=2c \\ a>b>c \\ m=\frac{a+b+c}{3}=21 \\ a+b+c-21\cdot 3 \\ 2b+2c+c=21\cdot 3 \\ 4c+2c+c=21\cdot 3 \\ 4\cdot c+2\cdot c+c=21\cdot 3 \\ 7c=63 \\ c=\frac{63}{7}=9 \\ c=9 \\ \text{ Verifying Solution: } \\ b=2\cdot c=2\cdot 9=18 \\ a=2\cdot b=2\cdot 18=36 \end{gathered}$$

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