Answer
$12cm$
Work Step by Step
We can find the required focal length as follows:
$f=(\frac{1}{d_i}+\frac{1}{d_{\circ}})^{-1}$
We plug in the known values to obtain:
$f=(\frac{1}{17}+\frac{1}{46})^{-1}$
$f=12cm$
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