Question 1167841
x ml of 11%
42-x ml of 18%
.11x+.18(42-x)=42*0.16=6.72 ml pure acid
.11x+7.56-.18x=6.72
-0.07x=-0.84
x=12 ml of 11%
42-x=30 ml of 18%

Also,
16% is 5/7 of the way from 11 to 18%
want 5/7 of the higher percent and 2/7 of the lower
with 42 ml, that is 30 ml of 18% and 12 ml of 12%.