\(\dfrac{1}{21}+\dfrac{2}{21}+\dfrac{3}{21}+\dfrac{4}{21}+\dfrac{5}{21}+\cdot\cdot\cdot+\dfrac{17}{21}+\dfrac{18}{21}+\dfrac{19}{21}+\dfrac{20}{21}\)
\(=\dfrac{1+2+3+4+5+\cdot\cdot\cdot+17+18+19+20}{21}\)
\(=\dfrac{210}{21}=10\)
1/21+2/21+3/21+4/21+5/21+...+17/21+18/21+19/21+20/21
\(=\dfrac{1+2+3+4+5+....+17+18+19+20}{21}\)
\(=\dfrac{210}{21}\)
\(=10\)