Half-life equations

Question

The half-life of radon-$222$ is $3.8$ days. if a sample contains $4.2$ grams, how much will remain in the sample after $15.2$ days?

I've tried solving it written out, but I keep getting the wrong answer.

Answer 1

Hint:

Use the equation $$N(t)=N_0e^{-\lambda t}$$ and $$\lambda=\frac{\ln2}{t_{\frac12}}$$ Half life time $t_{\frac12}=3.8$

$N_0=4.2$