\(\dfrac{7}{5}:x=\dfrac{25}{37}.\dfrac{13}{21}+\dfrac{2}{7}.\dfrac{13}{21}\)

\(\dfrac{7}{5}:x=\dfrac{13}{21}.\left(\dfrac{25}{37}+\dfrac{2}{7}\right)\)

\(\dfrac{7}{5}:x=\dfrac{1079}{1813}\)

\(x=\dfrac{12691}{5395}\)

a=\(1\times3+\dfrac{2}{3}\times7+\dfrac{3}{7}\times13+\dfrac{4}{13}\times21+\dfrac{5}{12}\times21\)

=3+\(\dfrac{14}{3}\)+\(\dfrac{39}{7}\)+\(\dfrac{84}{13}\)+5=8+\(\dfrac{215}{21}\)+\(\dfrac{84}{13}\)=\(\dfrac{4559}{273}\)+8=\(\dfrac{6743}{273}\)

= 48/385

1/4*7+1/4*9+1/5*9+1/5*11+1/6*11+1/6*13+1/7*13 = 1/4*(7+9)+1/5*(9+11)+1/6*(11+13)+1/7*13 = 1/4*16+1/5*20+1/6*24+13/7= 4+4+4+13/7 = 12+13/7 = 84/7+3/7 = 97/7

\(\frac{34}{7\times13}+\frac{51}{13\times22}+\frac{85}{22\times37}+\frac{68}{37\times49}\)

\(=17\times\left(\frac{2}{7\times13}+\frac{3}{13\times22}+\frac{5}{22\times37}+\frac{4}{37\times49}\right)\)

\(=\frac{17}{3}\times\left(\frac{6}{7\times13}+\frac{9}{13\times22}+\frac{15}{22\times37}+\frac{12}{37\times49}\right)\)

\(=\frac{17}{3}\times\left(\frac17-\frac{1}{13}+\frac{1}{13}-\frac{1}{22}+\frac{1}{22}-\frac{1}{37}+\frac{1}{37}-\frac{1}{49}\right)\)

\(=\frac{17}{3}\times\left(\frac17-\frac{1}{49}\right)=\frac{17}{3}\times\frac{6}{49}=\frac{17\times2}{49}=\frac{34}{49}\)