\[\boxed{\text{71\ (71).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ 3,17\ + \ 10,2\ + \ 0,83\ + \ 9,8\ = \ \]
\[= (3,17\ + \ 0,83)\ + \ (10,2\ + \ 9,8)\ = \ \]
\[= 4\ + \ 20\ = \ 24\]
\[\textbf{б)}\ 4,11\ + \ 15,5\ + \ 0,89\ + \ 4,4\ = \ \]
\[= (4,11\ + \ 0,89)\ + \ (15,5\ + \ 4,4)\ =\]
\[= \ 5\ + \ 19,9\ = \ 24,9\]
\[\textbf{в)}\ 15,21\ - \ 3,9\ - \ 4,7\ + \ 6,79\ = \ \]
\[= (15,21\ + \ 6,79) - \ (3,9\ + \ 4,7)\ = \ \]
\[= 22\ - \ 8,6\ = \ 13,4\]
\[\textbf{г)}\ - 4,27\ + \ 3,8\ - \ 5,73\ - \ 3,3\ = \ \]
\[= - (4,27\ + \ 5,73)\ + \ (3,8\ - \ 3,3)\ =\]
\[= \ - 10\ + \ 0,5\ = - (10 - 0,5) = \ \]
\[= - 9,5\]