Срочно! 10 баллов вместо 5!!! Номер 277, 278, 281. Не поняла тему. Помогите, мне очень...

$1)\frac{1}{4} + \frac{3}{5} = \frac{5+12}{20} = \frac{17}{20} \\ 2) \frac{9}{11} - \frac{2}{5} = \frac{45-22}{55} = \frac{23}{55} \\ 3) \frac{13}{16} - \frac{9}{32} = \frac{26-9}{32} = \frac{17}{32} \\ 4) \frac{3}{28}+ \frac{5}{14} = \frac{3+10}{28} = \frac{13}{28} \\ 5) \frac{14}{15} - \frac{7}{10} = \frac{28-21}{30} = \frac{7}{30} \\ 6) \frac{3}{8} + \frac{1}{6} = \frac{9+4}{24} = \frac{13}{24} \\ 7) \frac{9}{25} - \frac{7}{20} = \frac{36-35}{100} = \frac{1}{100} \\$ $8) \frac{37}{42} - \frac{17}{24} = \frac{148-119}{168} = \frac{29}{168} \\ 9) \frac{11}{24} - \frac{3}{16} = \frac{22-9}{48} = \frac{13}{48} \\ 10) \frac{9}{16} + \frac{7}{24} + \frac{3}{8}= \frac{27+14+18}{48} = \frac{59}{48} =1 \frac{11}{48} \\ 11) \frac{1}{3} - \frac{1}{6} + \frac{1}{4} = \frac{4-2+3}{12}= \frac{4}{12} = \frac{1}{3} \\ 12) \frac{2}{5} + \frac{4}{15} - \frac{5}{9} = \frac{18+12-25}{45} = \frac{5}{45} = \frac{1}{9}$
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$1) \frac{25}{80} + \frac{45}{60} = \frac{5}{16} + \frac{3}{4} = \frac{5+12}{16} = \frac{17}{16} =1 \frac{1}{16} \\ 2) \frac{20}{45} + \frac{26}{54} = \frac{4}{9} + \frac{13}{27} = \frac{12+13}{27} = \frac{25}{27} \\ 3) \frac{36}{300} + \frac{12}{40} - \frac{350}{1000} = \frac{6}{50} + \frac{3}{10} - \frac{7}{20} = \frac{12+30-35}{100} = \frac{7}{100} \\ 4) \frac{14}{24}- \frac{39}{90} + \frac{15}{100} = \frac{7}{12} - \frac{13}{30} + \frac{3}{20} = \frac{35-26+9}{60} = \frac{18}{60} = \frac{9}{30} \\$ $5) \frac{42}{120} + \frac{20}{32} - \frac{28}{160} = \frac{7}{20} + \frac{5}{8} - \frac{7}{40} = \frac{14+25-7}{40} = \frac{32}{40} = \frac{4}{5} \\ 6) \frac{45}{72} - \frac{33}{144} - \frac{20}{64} = \frac{5}{8} - \frac{11}{48} - \frac{5}{16} = \frac{30-11-15}{48} = \frac{4}{48} = \frac{1}{12}$
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$\frac{11}{20} - \frac{2}{15} = \frac{33-8}{60} = \frac{25}{60} = \frac{5}{12}$ часа затратила на мытье посуды
$\frac{11}{20} + \frac{5}{12} = \frac{33+25}{60} = \frac{58}{60} = \frac{29}{30}$ часа заняла уборка