Richard H. answered • 03/09/16

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A 4% solution has 4 pounds for every 96 pounds of water, and adding mass to seawater increases both salt content and overall mass, i.e. Additional 4 lbs of salt creates 8 lbs of total salt in 104 lbs solution or (4+4)/(100+4)=8/104=7.7% seawater.

To calculate a 40%, reverse the equation: 40% or 0.4 = (4+x)/(100+x) then expand 0.4(100+x) = 40+0.4x = 4+x and finally (1-0.4)x=40-4 leads to 0.6x=36

x=36/0.6=360/6=60 lbs of salt

Double check (4+60)/(100+60) = 64/160 = 0.4 or 40% seawater