Lens maker equation:

$\overline{)\frac{\mathbf{1}}{{\mathit{s}}_{\mathit{o}}}{\mathbf{+}}\frac{\mathbf{1}}{{\mathit{s}}_{\mathit{i}}}{\mathbf{=}}\frac{\mathbf{1}}{\mathit{f}}}$

Power of a lens:

$\overline{){\mathbf{D}}{\mathbf{=}}\frac{\mathbf{1}}{\mathbf{f}}}$

The focal length of the first lens is:

Oscar has an old-fashioned lantern projector that uses a candle and a lens to project a slide onto the wall. The lens is 1.2 cm from the slide and projects an image onto a screen 80 cm away. Oscar adds a lens to the original lens to allow him to project an image onto a screen 30 cm away.

What power lens must Oscar add?

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