The half-life of a radioactive substance is 10 days. This means that

The half life of radioactive substance is a characterized Constant. It measures the time it takes for a given amount of the substance to become reduced by half as a consequence of decay.

In 30 days (i.e. 3 half lives) the fraction of substance left is given by:

 $\frac{1}{2^3}=\frac{1}{8}$ of the initial substance. 

The fraction of substance decayed will be $1- \frac{1}{8} = \frac{7}{8}$of the initial substance.