Question 907530
We have 20 lb of 50% yarn
We want to add some 20% yarn to make   35% yarn
0.2*x+0.5*20=0.35(20+x)
0.2*x+10=7+0.35x
0.2x-0.35x=7-10
-0.15x=-3
x=20 lb at 20% (4) is added to 20 lb at 50%(10) to make 35% of 40 lb(14)
check
0.2*20+0.5*20=0.35(20+20)
4+10=0.35(40)
14=14
ok