Help with Geometric Means!

How do I find the geometric mean when there is an even number of terms (eg. 7 and 8 7 5, there are 4 terms)

$\begin{array}{rcll} \text{geometric mean } &=& \sqrt[n]{ x_1\cdot x_2\cdot x_3\cdot x_4 } \qquad x_1 = 7\qquad x_2 = 8\qquad x_3 = 7\qquad x_4 = 5\\ &=& \sqrt[4]{ 7\cdot 8\cdot 7\cdot 5 } \\ &=& \sqrt[4]{ 1960 } \\ \mathbf{ \text{geometric mean } } & \mathbf{=} & \mathbf{ 6.65371229032 } \end{array}\\$