Idk the answer to this really need help

Bradley hikes 12 miles in 3 hours. in relation to bradley’s hike, which statement describes the expression

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 (%)

I’m still confused on how to work the percentages