Answer:
558.036 lb
Explanation:
Let x is the amount of the sea water to collect 125 lb of sea salt 
As given: 64 lb/ft³ has 0.0035 dissolved salt by weight
So the formula to find out the amount of the sea water is: 
64 lb/ft³ * 0.0035 *x ft³ = 125 lb
<=> x = 
 = 558.036 lb 
Hope it will find you well.