X - 105 : 21 = 15

X    -     5     =15

X                 =15+5

X                 =20

\(C=\frac{2}{1.4}+\frac{2}{4.7}+\frac{2}{7.10}+...+\frac{2}{61.64}\)

\(C.\frac{3}{2}=\frac{3}{1.4}+\frac{3}{4.7}+\frac{3}{7.10}+...+\frac{3}{61.63}\)

\(C.\frac{3}{2}=1-\frac{1}{4}+\frac{1}{4}-\frac{1}{7}+\frac{1}{7}-\frac{1}{10}+...+\frac{1}{61}-\frac{1}{63}\)

\(C.\frac{3}{2}=1-\frac{1}{63}=\frac{62}{63}\)

\(C=\frac{62}{63}:\frac{3}{2}=\frac{124}{189}\)