81. Решите уравнение: 1) 6/5 x = 3/5; 2) 4/7 x = 1; 3) 3/4 x = 12; 4) 3x = 2/3; 5) x : 7/15 = 15/28; 6) 5 1/4 : x = 7/8.

Решим уравнение:

1. $$ \frac{6}{5} x = \frac{3}{5} $$
   $$ x = \frac{3}{5} : \frac{6}{5} $$
   $$ x = \frac{3}{5} \cdot \frac{5}{6} $$
   $$ x = \frac{3 \cdot 5}{5 \cdot 6} $$
   $$ x = \frac{1 \cdot 1}{1 \cdot 2} $$
   $$ x = \frac{1}{2} $$
2. $$ \frac{4}{7} x = 1 $$
   $$ x = 1 : \frac{4}{7} $$
   $$ x = 1 \cdot \frac{7}{4} $$
   $$ x = \frac{7}{4} $$
   $$ x = 1 \frac{3}{4} $$
3. $$ \frac{3}{4} x = 12 $$
   $$ x = 12 : \frac{3}{4} $$
   $$ x = 12 \cdot \frac{4}{3} $$
   $$ x = \frac{12 \cdot 4}{3} $$
   $$ x = \frac{4 \cdot 4}{1} $$
   $$ x = 16 $$
4. $$ 3x = \frac{2}{3} $$
   $$ x = \frac{2}{3} : 3 $$
   $$ x = \frac{2}{3} \cdot \frac{1}{3} $$
   $$ x = \frac{2}{9} $$
5. $$ x : \frac{7}{15} = \frac{15}{28} $$
   $$ x = \frac{15}{28} \cdot \frac{7}{15} $$
   $$ x = \frac{15 \cdot 7}{28 \cdot 15} $$
   $$ x = \frac{1 \cdot 1}{4 \cdot 1} $$
   $$ x = \frac{1}{4} $$
6. $$ 5 \frac{1}{4} : x = \frac{7}{8} $$
   $$ \frac{21}{4} : x = \frac{7}{8} $$
   $$ x = \frac{21}{4} : \frac{7}{8} $$
   $$ x = \frac{21}{4} \cdot \frac{8}{7} $$
   $$ x = \frac{21 \cdot 8}{4 \cdot 7} $$
   $$ x = \frac{3 \cdot 2}{1 \cdot 1} $$
   $$ x = 6 $$

Ответ:

1. $$x = \frac{1}{2}$$
2. $$x = 1 \frac{3}{4}$$
3. $$x = 16$$
4. $$x = \frac{2}{9}$$
5. $$x = \frac{1}{4}$$
6. $$x = 6$$