Б-2. а) 1,8 − 7,8 : 1 6/7; б) (3 1/7 − 2 1/3) : 1 13/21.

а)

\(1,8-7,8:1\frac{6}{7}=1,8-\frac{39}{5}:\frac{13}{7}\)

\(=1,8-\frac{39}{5}\cdot\frac{7}{13}=1,8-4,2=-2,4\)

б)

\(\left(3\frac{1}{7}-2\frac{1}{3}\right):1\frac{13}{21}=\left(\frac{22}{7}-\frac{7}{3}\right):\frac{34}{21}\)

\(=\frac{17}{21}:\frac{34}{21}=\frac{17}{21}\cdot\frac{21}{34}=\frac{1}{2}\)

Ответ: а) \(-2,4\); б) \(\frac{1}{2}\).