stop the compass

Dan Meyer poses the question:

you choose blue or purple

if  a spinner lands on the colour you chose you win that amount









which region would you pick for this picture?

• if the spinner stops on purple and you have chosen it, you get $200 
• if it stops on blue and you have chosen it you get $100
• if it stops on the other colour to the one you chose, you lose

an image from Dan (on Vimeo) sets up the problem:

where/when would you stop choosing the purple?