It's a weighted average

x = number of lbs of 35% yarn needed

20 lbs of yarn*0.5 wool + x lbs of yarn*0.35 wool = number of pounds of wool in rug

20 + x = total weight of rug

You want (number of pounds of wool in rug)/ (total weight of rug) = 0.35

So if you solve this you get 30 lbs