Given: Half life of radioactive element $t_{\frac{1}{2}}=30 \min$.

In $90 \min$, there will be three half lives, so $n=3$.

Using radioactive decay law, fraction of radioactive element remains undecayed is 

$\frac{N}{N_0}={\left(\frac{1}{2}\right)}^n={\left(\frac{1}{2}\right)}^3$

$\Rightarrow \frac{{\displaystyle N}}{{\displaystyle N_0}}=\frac{1}{8}$