Task I: 20-15-30

The sizes of the green and brown angles are given. Prove that the shown value of the red angle is correct.

You can download a pdf version of the diagram from here.

INTRO10-10-2010-10-3010-15-2010-15-5510-20-3010-20-5010-30-4010-30-6020-15-3020-15-3510-20-45