Hi, i am studying maths on khan academy and got stuck with a certain problem and i can't find any videos which explain it so i could do with some help please.

When i look up how to do this problem on the internet it says i just do 321/360 * the circumference. simple but i want to know how to work it out the way they have done it in the image. I don't understand how he gets from step 2 to step 3, how he gets the 321/360 to 107/6. so the question isn't really geometry but i didn't know where else to put it.

"A circle with circumference20πhas an arc with a321∘central angle."What is the length of the arc?