What is the solution set of the equation ?

School has 204 sixth grade students. if the sixth grade is 30% of the total school, how many students are in the middle school?

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 the following pair of equations for x and y: ax/b_ by /a= a +b; ax - by = 2ab