Caffeine has a molecular weight of 194. If it contains 28.9% by mass of nitrogen, number of atoms of nitrogen in one molecule of caffeine is 

   100 g caffeine has 28.9 g nitrogen

$\begin{array}{l}194\mathrm{g}\mathrm{caffeine}\mathrm{has}=\frac{28.9}{100}\times 194=56.06\mathrm{g}\\ \therefore \mathrm{No}.\mathrm{of}\mathrm{atoms} \mathrm{of} \mathrm{nitrogen} \mathrm{in} \hspace{0.17em}\mathrm{one} \mathrm{molecule} \mathrm{of} \mathrm{caffeine}=\frac{56.06}{14}\approx 4.\end{array}$