Number of atoms present in 4.25g of NH3 is:

No. of moles of ammonia = 4.25/17 = 0.25 molNo. of molecules of ammonia =  6.023×1023×0.25One molecule contains a total 4 atoms.Thus, total no. of atoms in ammonia =6.023×1023×0.25×4=6.023×1023