Answer
$f_o = 2.7~mm$
Work Step by Step
We can find $f_o$:
$M = \frac{L~D}{f_o~f_e}$
$f_o = \frac{L~D}{M~f_e}$
$f_o = \frac{(16~cm)(6.0~mm)}{(40)~(9.0~mm)}$
$f_o = 0.27~cm$
$f_o = 2.7~mm$
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