s = standard version amount
h = high quality version amount
we know that there were 1090 downloads of the song, meaning s + h = 1090.
we also know that the total amount of MBs downloaded was 3353 MBs, and since the standard is 2.1 MBs and the high quality is 4.9MBs, then 2.1s + 4.9h = 3353.
[tex]\begin{cases} s+h=1090\\ 2.1s + 4.9h = 3353\\[-0.5em] \hrulefill\\ h = 1090 - s \end{cases}\qquad \stackrel{\textit{substituting on the 2nd equation}}{2.1s+4.9(1090-s) = 3353} \\\\\\ 2.1s + 5341 - 4.9s = 3353\implies -2.8s + 5341 = 3353 \\\\\\ -2.8s=-1988\implies s = \cfrac{-1988}{-2.8}\implies \boxed{s = 710}[/tex]
