Find the missing side
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Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much

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 (%)

Could somebody me with this radical simplification?