Question

If desalinated water costs $ 1900 per acre-foot, how much does desalinated water cost per liter?

Answer 1

Note that
1 acre-foot = 1233481.84 L
or 
1 L = 1/1233481.84 acre-foot = 8.1071 x 10⁻⁷ acre-foot

Desalinated water costs
($1900 1/acre-foot)*(8.1071 x 10⁻⁷ acre-foot/L) = $1.54 x 10⁻³ per liter

Answer:  $1.54 x 10⁻³ per lter