let's firstly convert the mixed fractions to improper fractions and then add them up.
[tex]\bf \stackrel{mixed}{1\frac{11}{12}}\implies \cfrac{1\cdot 12+11}{12}\implies \stackrel{improper}{\cfrac{23}{12}}~\hfill \stackrel{mixed}{1\frac{1}{3}}\implies \cfrac{1\cdot 3+1}{3}\implies \stackrel{improper}{\cfrac{4}{3}} \\\\[-0.35em] ~\dotfill[/tex]
[tex]\bf \stackrel{\textit{Alex's}}{\cfrac{4}{3}}~~+~~\stackrel{\textit{more yards}}{\cfrac{23}{12}}\implies \stackrel{\textit{using an LCD of 12}}{\cfrac{(4)4~~+~~(1)23}{12}}\implies \cfrac{16+23}{12} \\\\\\ \cfrac{39}{12}\implies \stackrel{\textit{simplified}}{\cfrac{13}{4}}\implies 3\frac{1}{4}[/tex]
