Solving a value mixture problem using a linear equation

 A Web music store offers two versions of a popular song. The size of the standard version is 

2.1

megabytes (MB). The size of the high-quality version is 

4.5

MB. Yesterday, the high-quality version was downloaded twice as often as the standard version. The total size downloaded for the two versions was 

2553

MB. How many downloads of the standard version were there?