We can use the lens formula:
$\frac{1}{f_1} = \frac{1}{d_o} + \frac{1}{d_i}$
where $f_1 = 20\,\text{cm}$ is the focal length of the first lens, $d_o = 40\,\text{cm}$ is the object distance, and $d_i$ is the image distance. Plugging in the values, we
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