bearing in mind that the LCD in this case for those two denominators is 40, so let's use that for the sum
[tex]\bf \cfrac{18}{20}+\cfrac{3}{8}\implies \stackrel{\textit{using the LCD of 40}}{\cfrac{(2)18+(5)3}{40}}\implies \cfrac{36+15}{40}\implies \cfrac{51}{40}\implies 1\frac{11}{40}[/tex]
