Вариант Б1 Найдите значение выражения: а) (5,2-3):1 б) (+21 7 20

a)

$$5,2 - 3\frac{1}{3} = 5\frac{2}{10} - 3\frac{1}{3} = 5\frac{1}{5} - 3\frac{1}{3} = \frac{26}{5} - \frac{10}{3} = \frac{26 \cdot 3 - 10 \cdot 5}{15} = \frac{78 - 50}{15} = \frac{28}{15}$$

$$\frac{28}{15} : 1\frac{1}{3} = \frac{28}{15} : \frac{4}{3} = \frac{28}{15} \cdot \frac{3}{4} = \frac{28 \cdot 3}{15 \cdot 4} = \frac{7 \cdot 1}{5 \cdot 1} = \frac{7}{5} = 1\frac{2}{5}$$

б)

$$\frac{3}{8} + 2\frac{1}{7} \cdot \frac{1}{20} = \frac{3}{8} + \frac{15}{7} \cdot \frac{1}{20} = \frac{3}{8} + \frac{15}{7 \cdot 20} = \frac{3}{8} + \frac{3}{7 \cdot 4} = \frac{3}{8} + \frac{3}{28} = \frac{3 \cdot 7 + 3 \cdot 2}{56} = \frac{21 + 6}{56} = \frac{27}{56}$$

$$\frac{27}{56} \cdot \frac{1}{3} = \frac{27}{56 \cdot 3} = \frac{9}{56}$$

Ответ: a) $$1\frac{2}{5}$$, б) $$\frac{9}{56}$$