12.
[tex]\frac{2}{3}\cdot1,2 \ kg = 0,8 \ kg\\\\1,2 \ kg - 0,8 \ kg = 0,4 \ kg\\\\\frac{1}{5}\cdot0,4 \ kg = 0,08 \ kg =80 \ g[/tex]
Odp. Ten kawałek tortu ważył 80 gramów.
13.
[tex]a) \ (0,5+0,3\cdot\frac{1}{2}):10 =(\frac{1}{2}+\frac{3}{10}\cdot\frac{1}{2}):10=(\frac{10}{20}+\frac{3}{20}):10 = \frac{13}{20}\cdot\frac{1}{10} = \frac{13}{200}[/tex]
[tex]b) \ (5-2\frac{2}{9})\cdot(3,5-2,6) = (4\frac{9}{9}-2\frac{2}{9})\cdot0,9 =2\frac{7}{9}\cdot\frac{9}{10} = \frac{25}{9}\cdot\frac{9}{10} = \frac{5}{2} = 2\frac{1}{2} = 2,5[/tex]
[tex]c) \ \frac{22-3\cdot4}{5+3\cdot9}:1\frac{1}{4} = \frac{22-12}{5+27}:\frac{5}{4} = \frac{10}{32}\cdot\frac{4}{5} = \frac{2}{8} = \frac{1}{4}[/tex]
[tex]d) \ \frac{3,4-1,8}{0,8}\cdot\frac{\frac{1}{6}+\frac{3}{4}-0,5}{2,4:0,6} = \frac{1,6}{0,8}\cdot\frac{\frac{2}{12}+\frac{9}{12}-\frac{1}{2}}{4}=2\cdot\frac{1}{4}\cdot(\frac{11}{12}-\frac{6}{12}) = \frac{1}{2}\cdot\frac{5}{12} = \frac{5}{24}[/tex]
