How should a scarf with 70 percent viscosa and 30 percent seta be cleaned?

If you have 70% of something and 30% of another identical "something" that is equivalent to 50% of the two "somethings".

30% of 30% = 0.09 or 9%

27.3 is the answer. But you can round it off to 27.

the answer is 42.9 % 30 X 100 = 3000 3000 : 70 = 42.85714286

(30/100) X 70 feet = 21 feet.