Aweb 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 4.2 mb. yesterday, the high-quality version was downloaded twice as often as the standard version. the total size downloaded for the two versions was 2205 mb. how many downloads of the standard version were there?

This is the concept of algebra, suppose x standard quality were downloaded. The number of high quality downloaded was 2x.
The to total size of standard quality downloaded was:
2.1*x=2.1x mbs
The total size high quality downloaded was
4.2*2x=8.4x mbs
thus the total number of mbs downloaded will be:
2.1x+8.4x=2205
solving for x we get
10.5x=2205
x=2205/10.5
x=210
this means that 210 standard quality music were downloaded

This task can be solved with simple equations.
Set the values:
x is the number of standard downloady is number of high-quality download = 2x

Then set up the equation for total size downloaded: 2.1x + 4.2y =2205
Then substitute the values (y=2x), as you know that the high-quality version was downloaded twice as often as the standard version: 
2.1x + (4.2*2x) = 220510.5x = 2205x = 2205:10.5x
x= 210

 The standard version was downloaded 210 times.