Pls step by step clearly. Please step by step with writing explanation.

A solution containing 20% by weight of NaOH with a temperature of 311 K in a standard type evaporator is evaporated until its concentration (by weight) reaches 50%. The temperature of the Saturated Water vapor used as the heating medium is 398 K and there is an absolute pressure of 12.3 kPa in the separator part of the evaporator. Since the entire heat transfer coefficient of the system is 1420 watts/mK, the specific heat of the overheated solvent vapor is 1.926 kj/kg.K and heat transfer area is 86.4 m².

a) Calculate the flow rate of the feed stream b) Calculate the evaporator efficiency