let's firstly convert the mixed fractions to improper, keeping in mind that mark drank some in the morning and some in the afternoon, so his total is 3⅖ + 2⅕.
[tex] \bf \stackrel{mixed}{3\frac{2}{5}}\implies \cfrac{3\cdot 5+2}{5}\implies \stackrel{improper}{\cfrac{17}{5}}
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\stackrel{mixed}{2\frac{1}{5}}\implies \cfrac{2\cdot 5+1}{5}\implies \stackrel{improper}{\cfrac{11}{5}}\\\\
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\cfrac{17}{5}+\cfrac{11}{5}\quad \stackrel{LCD~of~5}{\implies }\quad \cfrac{17+11}{5}\implies \cfrac{28}{5}\implies 5\frac{3}{5} [/tex]
