Răspuns:
a = 15
Explicație pas cu pas:
[tex]2a - \frac{60}{100} *a=21\\\\\frac{200}{100}a- \frac{60}{100} *a =21\\\\\frac{140}{100}a=21\\\\\\ \frac{14}{10}a=21\\\\ \frac{7}{5}a=21\\\\ a=21:\frac{7}{5}\\\\ a=\frac{21}{1}:\frac{7}{5}\\\\ a=\frac{21}{1}*\frac{5}{7}\\\\ a=\frac{3}{1}*\frac{5}{1}\\\\ a=15[/tex]