a) $11+\left(-12\right)+39+9-8=\left(11+39+9\right)+\left[\left(-12\right)+\left(-8\right)\right]=59+\left(-20\right)=59-20=39$

b) $45+\left(-9\right)+\left(-91\right)+5=\left(45+5\right)+\left[\left(-9\right)+\left(-91\right)\right]=50+\left(-100\right)=50-100=-50$

c)  $-4\frac{1}{2}+\frac{1}{6}+\left(-\frac{1}{2}\right)+1\frac{1}{3}=$ $\left(\frac{1}{6}+1\frac{1}{3}\right)+\left[\left(-4\frac{1}{2}\right)+\left(-\frac{1}{2}\right)\right]=$ $\left(\frac{1}{6}+1\frac{2}{6}\right)+\left(-5\right)=1\frac{3}{6}+\left(-5\right)=1\frac{1}{2}-5=-3\frac{1}{2}$

d)  $6,25+\left(-\frac{1}{4}\right)+\left(-\frac{3}{5}\right)+0,75=$ $\left(6,25+0,75\right)+\left[\left(-\frac{5}{20}\right)+\left(-\frac{12}{20}\right)\right]=$ $7+\left(-\frac{17}{20}\right)=7-\frac{17}{20}=6\frac{3}{20}$

e)  $\frac{4}{7}+\left(-\frac{3}{14}\right)+\frac{1}{14}=\frac{8}{14}+\frac{1}{14}+\left(-\frac{3}{14}\right)=$ $\frac{9}{14}-\frac{3}{14}=\frac{6}{14}=\frac{3}{7}$

f)  $\frac{1}{2}+\left(-\frac{3}{4}\right)+\left(-\frac{5}{8}\right)=\frac{1}{2}+\left[\left(-\frac{6}{8}\right)+\left(-\frac{5}{8}\right)\right]=$ $\frac{4}{8}+\left(-\frac{11}{8}\right)=\frac{4}{8}-\frac{11}{8}=-\frac{7}{8}$

g)  $5\frac{2}{3}+\left(-3\frac{4}{5}\right)+\frac{1}{3}+\left(-\frac{1}{5}\right)=$ $\left(5\frac{2}{3}+\frac{1}{3}\right)+\left[\left(-3\frac{4}{5}\right)+\left(-\frac{1}{5}\right)\right]=6+\left(-4\right)=6-4=2$

h)  $16\frac{4}{5}+\left(-4,8\right)+3\frac{1}{5}+\left(-4\frac{1}{5}\right)=$ $\left(16\frac{4}{5}+3\frac{1}{5}\right)+\left[\left(-4\frac{4}{5}\right)+\left(-4\frac{1}{5}\right)\right]=$ $20+\left(-9\right)=20-9=11$