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Of the three numbers, the first is twice the second and the second is twice the third. The average of the three numbers is 21. The smallest of the three numbers is:

1. 6
2. 9
3. 12
4. 18

Correct answer: 9

Solution

Let the third number be \(x\). Then the second is \(2x\) and the first is \(4x\). Their average is \(\frac{4x+2x+x}{3}=21\), so \(7x=63\) and \(x=9\). The smallest number is 9.

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