Answer
$9.1mm$
Work Step by Step
As $d_i=d-f_{eye\space }$
$\implies d_i=15.0-5.0=10.00cm$
Now we can find the focal length of the objective lens as follows:
$f_{objective}=(\frac{1}{d}+\frac{1}{d_{\circ}})^{-1}$
We plug in the known values to obtain:
$f_{objective}=(\frac{1}{10}+\frac{1}{1.0})^{-1}$
$f_{objective}=9.1mm$
