Desalination of brackish water is achieved by evaporation. A fraction of feed water is sent through an evaporator and the other is bypassed. The fraction bypassed and the pure water leaving the evaporator are mixed together to give the desalinasied water with desired quality. If the feed water has 4000 ppm of salt and the salt content in the water used for irrigation is to be limited to a maximum of 50 ppm, what fraction of the feed water is to be sent through the evaporator