A particle of mass ' $m$ ' is moving in time ' $t$ ' on a trajectory given by $$ \overrightarrow{\mathrm{r}}=10 \alpha t^2 \hat{\mathrm{i}}+5 \beta(\mathrm{t}-5) \hat{\mathrm{j}} $$ Where $\alpha$ and $\beta$ are dimensional constants. The angular momentum of the particle becomes the same as it was for $t=0$ at time $t=$ $\_\_\_\_$ seconds.