What is the answer to 20x+5y=-30

There were 816 people at a concert when a band started to play. after each song, only half the people stayed to hear the next song. how many people were left at the concert after x songs? write a function to represent this scenario.

You are cycling around europe with friends. a cycling festival is starting soon in barcelona. it will take 4.5 hours to cycle there. how many minutes in total is this?

What is the point of inflection for the function f(x)=2cos4x on the interval (0, pi/2)