Here's a simple geometric puzzle that I happen to come across some time ago. Without cheating (such as constucting the figure and measuring the angle to be solved), how long does it take for you to solve it mathematically?
Added rule: without using trigo functions like sin/cos/tan etc...
Please post your timing (hours, days, weeks, months....) and the angle, but not the solution.
Note, the two 80 deg marking refers to the two inner angles of the base of the main triangle. The question is to solve for the angle marked with a "?".