$($a$)$Why desalting of crude oil is essential during surface processing? $(2$ Marks$)$

$($b$)$ A crude oil dehydrator outlet stream is diluted $10\%$ by volume by mixing freshwater of salinity $1000ppm.$ The water content in the dehydrator outlet stream is $15\%$ by volume and salinity is $15000ppm.$ If the mixing efficiency is $80\%,$ estimate the resulting salinity of after mixing. $(3$ Marks$)$