The equation 1.5r + 15 = 2.25r represents the number r of movies you must rent to spend the same amount at each movie store. how many movies must you rent to spend the same amount at each movie store?

1.5r+15=2.25r
First of all, you have to arrange the numbers according to those similar to them. So that will be;
15=2.25r-1.5r
1.5r is subtracted from 2.25r because it crossed over an equality sign to get its position close to 2.25r. And when a number crosses over an equality sign, the sign on the number chages to the opposite. 1.5r has an invisible positive sign which became negative when it crossed. Anyway;
2.25r-1.5r is the same as 2.25-1.5. And that is 0.75r
  Therefore;
15=0.75r
To find out what r is, you have to divide the both sides by 0.75. This is done to remove the 0.75 close to r to finally reveal what r is. Anyway;
15/0.75=0.75r/0.75
15 divided by 0.75 is 20. And 0.75r divided by  0.75 is r. So;
20=r
r=20. So the answer is 20 movies. Hope i helped. Have a nice day.