From the problem, we know that $d_o = 30.0 \mathrm{cm}$ and $d_i = -10.0 \mathrm{cm}$ (negative because the image is on the opposite side of the object).
Plugging these values into the equation, we get:
$\frac{1}{30.0 \mathrm{cm}} + \frac{1}{-10.0 \mathrm{cm}}
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