What is the relative formula mass of ammonium nitrate, NH4NO3?

Relative formula mass is the mass compared  of the compound compared with the mass of atoms of other elements. 
Using the periodic table, the individual Ar (relative atomic mass) can help you find this answer!
Make sure if there is more than one of each element in the formula, you multiply by the number of times it appears. 

Ar of Nitrogen (N) = 14 (x2) --> N in NH4 & N in NO3
Ar of Hydrogen (H) = 1 (x4) --> Hs in H4 
Ar of Oxygen (O) = 16 (x 3) --> Os in O3 Answer = 80