This is the boss. No calculator — every reciprocal on this bench was chosen to be kind ($\tfrac{1}{30} + \tfrac{1}{15}$ kind), and the rest is multiplying small whole numbers. One candle, one lens, one screen, and every answer feeds the next: the focal length, then the image's new position when the candle moves, then the magnification, then the height of the flame on the screen. Write each line down as you go — one early slip follows you down the whole bench.

Everything from the chapter rides in it. 5★ under par here is the chapter crown. Light the candle.