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What properties allow transformation to be used as a problem solving tool

Acompany that manufactures and sells guitars made changes in their product range. from the start they had 20 models. then they reduced the number of models to 15. as a result, the company enjoyed a 10% increase in turnover. - how much did the stock level change? in (%)

Solve each quadratic equation by factoring and using the zero product property. x^2 - 8x + 30 = 3x