An oculist charges $30 for an eye exam, frames, and glass lenses, but $42 for an eye exam, frames, and plastic lenses. If plastic lenses cost four times as much as glass lenses, how much do glass lenses cost?
An oculist charges $30 for an eye exam, frames, and glass lenses, but $42 for an eye exam, frames, and plastic lenses. If plastic lenses cost four times as much as glass lenses, how much do glass lenses cost?