$(7\frac{5}{19}+3\frac{7}{24})-<!--\; -\; -->4\frac{5}{19}=(7\frac{5}{19}-<!--\; -\; -->4\frac{5}{19})+3\frac{7}{24}=3+3\frac{7}{24}=6\frac{7}{24}$

$(13\frac{2}{17}+6\frac{12}{35})-<!--\; -\; -->5\frac{12}{35}=(6\frac{12}{35}-<!--\; -\; -->5\frac{12}{35})+13\frac{2}{17}=1+13\frac{2}{17}=14\frac{2}{17}$

$12\frac{11}{21}-<!--\; -\; -->(3\frac{2}{13}+8\frac{11}{21})=(12\frac{11}{21}-<!--\; -\; -->8\frac{11}{21})-<!--\; -\; -->3\frac{2}{13}=4-<!--\; -\; -->3\frac{2}{13}=3\frac{13}{13}-<!--\; -\; -->3\frac{2}{13}=\frac{11}{13}$

$20\frac{21}{43}-<!--\; -\; -->(9\frac{21}{43}+2\frac{15}{37})=(20\frac{21}{43}-<!--\; -\; -->9\frac{21}{43})+2\frac{15}{37}=11+2\frac{15}{37}=13\frac{15}{37}$