use geometric mean to calculate computer performance

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how to use geometric mean to calculate computer performance? the 2.2 units The performance of computers COM A and COM B was compared by benchmark using three programs. When geometric mean is used as the evaluation scale, The performance of COM B was twice that of COM A (COM B has better performance). What is the execution time of program 3 in COM B?

program | Execution time on COM A (seconds) | Execution time on COM B(sec)
1       |    12                             |9
2       |     6                             |3
3       |    24                             | ??

the root is 3 i thought becuase COM B is 2 times than COM A then $\frac{\sqrt[3]{9.3.x}}{\sqrt[3]{12.6.24}}=\frac{2}{1}$

but why is the right answer $\frac{\sqrt[3]{9.3.x}}{\sqrt[3]{12.6.24}}=\frac{1}{2}$? why is that(?)