208 - (6 x 21) = 82
You need to decrease out 82 sts.
208/82 = 2.5
My answer: Your decreases have to be less than every 3 sts but more often than every 2 sts.
How to space them?
Divide it into 4ths. 82/4 = 20.5
Take out 20 in each quarter.
I think this time it's figuring out how often to skip a decrease.
I hate story problems. I had to give it a try. Someone else can give you a real answer. I kind of want to see if my method is anywhere near what's needed.