Derive, based on a material balance, an equation that allows one to calculate the concentration in the raffinate phase after n extractions with the same volume of solvent. How many extractions are needed to extract $99\%$ of a hydrocarbon with an organic solvent from a starting phase, given that $50$ ml of fresh solvent is used each time and there is $1000$ ml of starting phase? The Nernst distribution coefficient is $30.$