Half life of a first order reaction is 4s and the initial concentration of the reactant is 0.12 M. The concentration f the reaction left after 16s is  

$t_{1/2}=4sT=16s$

$n=\frac{T}{t_{1/2}}=\frac{16}{4}=4 (\therefore T=n\times t_{1/2})$

$\left[A\right]={\left[A\right]}_0{\left(\frac{1}{2}\right)}^n=0.12\times {\left(\frac{1}{2}\right)}^4=\frac{0.12}{16}=0.0075M$

where ${\left[A\right]}_0$= initial concentration and [A] = concentration left after time t