How to use Half Life Calculator?

Problems :

300g of an unknown radioactive substance decays to 112g after 180 seconds.
What is the half life of this substance?

Solution :

$$\text{N}_0\;=\;300g$$

$$\text{t}\;=\;180s$$

$$\text{N}\;=\;112g$$

Formula :

$$\text{t}_\frac{1}{2}\;=\;\frac{t}{\text{Log}_\frac{1}{2}\left(\frac{\text{N}\left(t \right)}{\text{N}_0}\right)}$$

$$\text{t}_\frac{1}{2}\;=\;\frac{t}{\text{Log}_\frac{1}{2}\left(\frac{112g\left(t \right)}{300g}\right)}$$

$$\text{t}_\frac{1}{2}\;=\;127\text{s}$$