well, first off, you convert the mixed fraction to "improper", 7*5+3 over 5.
and then you get the LCD... and the subtraction is just the same as adding, you're just adding per se a negative value, is all.
[tex]\bf 7\frac{3}{5}-\cfrac{1}{2}\implies \cfrac{7\cdot 5+3}{5}-\cfrac{1}{2}\implies \cfrac{38}{5}-\cfrac{1}{2}\impliedby \textit{so our LCD is 10}
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\cfrac{(2\cdot 38)-(5\cdot 1)}{10}\implies \cfrac{76-5}{10}\implies \cfrac{71}{10}
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71\div 10\textit{ gives 7, with a \underline{remainder} of 1}\implies \cfrac{71}{10}\implies 7\frac{1}{10}[/tex]
