[tex]w = \frac{m-M}{M} *100 \ \ \ \ \ \ \ \ \ /*M[/tex]
[tex]wM=m-M*100 \ \ \ \ \ \ \ \ \ \ /:100[/tex]
[tex]m - M=\frac{wM}{100} \ \ \ \ \ \ \ \ /+M[/tex]
[tex]m = \frac{wM}{100} +M[/tex]
