Resposta :
Supomos que esse certo número seja "k", então: 31% . k = 2015
[tex]\frac{31}{100}\cdot k=2015\\\\\frac{31k}{100}=2015\\\\31k=201500\\\\\boxed{k=6500}[/tex]
[tex]\frac{31}{100}\cdot k=2015\\\\\frac{31k}{100}=2015\\\\31k=201500\\\\\boxed{k=6500}[/tex]