On average, seawater in the world oceans has a salinity of about 3.5%. How much seawater need to have evaporated to leave 100g salt?

The answer is:  " 2,857. 143 g  seawater " .
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Explanation:
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(3.5 parts salt) /(100 parts water) =  (100g salt)/ ("x" grams seawater) ;

Solve for "x" ,  {the amount of seawater, in "grams" (g), needed to have                                               evaporated to leave 100 g salt}.

Cross multiply:

100* 100 = (3.5)x ;

10,000 = (3.5)x ;

↔  (3.5)x = 10,000 ;

Divide each side of the equation by "(3.5)" ;  to isolate "x" on one side of the equation ; & to solve for "x" ; 

→ (3.5)x / (3.5) = (10,000) / (3.5) ;

             → x = (10,000) / (3.5) ;

             → x = 2857.1428571428571429 ; → round to:  2,857. 143 ; 

                  → round to:  " 2,857. 143 g  seawater " .
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