The population of Alaska was about 128,535. That was about 20.5% about how many people lived there?

Question

how would the proportion go?

20.5/100 = x/128535
or
20.5/100 = 128535/x?

Answer 1

I would start with
128535/20.5 = x/100
now multiply by 100 to get
x = 627 000

BTW, the wording of your question makes little sense, please check it.