How do you know the relationship shown is proportional? What is the constant of proportionality? What is the slope? Write the equation of the line?

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 (%)