\(\dfrac{5}{7}\times\dfrac{4}{15}+\dfrac{5}{7}\times\dfrac{11}{15}+\dfrac{2}{7}\\ =\dfrac{5}{7}\times\left(\dfrac{4}{15}+\dfrac{11}{15}\right)+\dfrac{2}{7}\\ =\dfrac{5}{7}\times1+\dfrac{2}{7}\\ =\dfrac{5}{7}+\dfrac{2}{7}\\ =\dfrac{7}{7}\\ =1\)

\(\dfrac{5}{7}\times\dfrac{4}{15}+\dfrac{5}{7}\times\dfrac{11}{15}+\dfrac{2}{7}\\ =\dfrac{5}{7}\times\left(\dfrac{4}{15}+\dfrac{11}{15}\right)+\dfrac{2}{7}\\ =\dfrac{5}{7}\times1+\dfrac{2}{7}\\ =\dfrac{5}{7}+\dfrac{2}{7}\\ =\dfrac{7}{7}\\ =1\)

\(a,13,25:0,75+13,75:0,25+13,75:0,325\\ =13,25:\left(0,75+0,25+0,325\right)\\ =13,25:1,325\\ =10\\ b,\dfrac{1}{4}:0,25-\dfrac{1}{8}:0,125+\dfrac{1}{2}:0,5-\dfrac{1}{16}:0,0625\\ =\dfrac{1}{4}:\dfrac{1}{4}-\dfrac{1}{8}:\dfrac{1}{8}+\dfrac{1}{2}:\dfrac{1}{2}-\dfrac{1}{16}:\dfrac{1}{16}\\ =1-1+1-1\\ =0\)

\(1,05=1\dfrac{1}{20}=\dfrac{21}{10}\)

\(\left(\dfrac{3}{4}+\dfrac{1}{6}\right)\times\left(\dfrac{5}{9}-\dfrac{1}{6}\right)\\ =\left(\dfrac{9}{12}+\dfrac{2}{12}\right)\times\left(\dfrac{10}{18}-\dfrac{3}{18}\right)\\ =\dfrac{11}{12}\times\dfrac{7}{18}\\ =\dfrac{77}{216}\)

\(2,05\times4,78+2,05\times5,22\\ =2,05\times\left(4,78+5,22\right)\\ =2,05\times10\\ =20,5\)