\[\boxed{\text{7.51.}}\]
\[\textbf{а)}\ 41,354\ :m + 170,79\ :n\]
\[m = 100;\ \ n = 10:\]
\[41,354\ :100 + 170,79\ :10 =\]
\[= 0,41354 + 17,079 = 17,49254\]
\[\textbf{б)}\ 633,74x - 9878\ :y\]
\[x = 100;\ \ y = 1000:\]
\[633,74 \cdot 100 - 9878\ :1000 =\]
\[= 63\ 374 - 9,878 = 63\ 364,122\]
\[c = 0,3:\]
\[= 13,14 + 75,6 = 88,74\]