a. $\frac{7}{11}$ + $\frac{3}{4}$ + $\frac{4}{11}$ + $\frac{1}{4}$ = ($\frac{7}{11}$ + $\frac{4}{11}$) + ($\frac{3}{4}$ + $\frac{1}{4}$) = 1 + 1 = 2
b. $\frac{72}{99}$ - $\frac{28}{99}$ - $\frac{14}{99}$ = $\frac{72}{99}$ - ($\frac{28}{99}$ + $\frac{14}{99}$) = $\frac{72}{99}$ - $\frac{42}{99}$ = $\frac{30}{99}$ = $\frac{10}{33}$
c. 69,78 + 35,97 + 30,22 = ( 69,78 + 30,22) + 35,97 = 100 + 35,97 = 135,97
d. 83,45 - 30,98 - 42,47 = 83,45 - (30,98 + 42,47) = 83,45 - 73,45 = 10