World's hardest easy geometry problem

Task C: 10-15-20

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

Do you have a possible proof?

Send it to info@mathsmed.co.uk
or tweet @ProfSmudge