A paperweight consists of a 9.00-cm-thick plastic cube. Within the plastic a thin sheet of paper is embedded, parallel to opposite faces of the cube. On each side of the paper is printed a different joke that can be read by looking perpendicularly straight into the cube. When read from one side (the top), the apparent depth of the paper in the plastic is 4.00 cm. When read from the opposite side (the bottom), the apparent depth of the paper in the plastic is 1.63 cm. What is the index of refraction of the plastic?