Exercise 1 solution

angle(36, 35)   = 54.4 degrees (full frame)
angle(23.7, 35) = 37.4 degrees (APS-C)

The full-frame view is wider. The APS-C sensor crops the view.
Crop factor for 23.7 x 15.6 mm = 43.3 / 28.4 = about 1.52, so the match is 35 x 1.52 = about 53 mm on full frame (about 52 to 53 mm; using the rounded 1.5 gives 52.5).