4. Вычислите: а) 1-3,21 + 1-1,91 – 12,251; б) $1\frac{17}{48}: 1-2\frac{5}{6}$.

a) $1 - 3,21 + 1 - 1,91 - 12,251 = -3,21 - 1,91 - 12,251 + 2 = -15,371$ б) $1\frac{17}{48}: 1-2\frac{5}{6}$ =$\frac{65}{48}:\frac{11}{6}$ =$\frac{65}{48}*\frac{6}{11}$ =$\frac{65}{8}*\frac{1}{11}$ = $\frac{65}{88}$