3x/20 = 21/5

3x.5 = 21.20

15x = 420

x = 420 : 15

x = 28

2,3-(-2)+(-2,3)+7,8+20%

=2,3-2,3+2+7,8+0,2

=2+8

=10

80

= 80 nhaa

\(\dfrac{2}{3}-\left(-\dfrac{5}{7}+\dfrac{2}{3}\right)=\dfrac{2}{3}+\dfrac{5}{7}-\dfrac{2}{3}=\dfrac{5}{7}\)

\(\dfrac{14}{13}+\left(\dfrac{-1}{13}-\dfrac{19}{20}\right)=\dfrac{14}{13}-\dfrac{1}{13}-\dfrac{19}{20}\)

\(=\dfrac{13}{13}-\dfrac{19}{20}=1-\dfrac{19}{20}=\dfrac{20}{20}-\dfrac{19}{20}\)

\(=\dfrac{1}{20}\)

=\(\dfrac{14}{13}\) + ( \(\dfrac{-20}{260}\) - \(\dfrac{247}{260}\) )

=\(\dfrac{14}{13}\) + \(\dfrac{-267}{260}\)

=\(\dfrac{280}{260}\) + \(\dfrac{-267}{260}\)

=\(\dfrac{13}{260}\)

=\(\dfrac{1}{20}\)

 

\(\dfrac{-13}{15}+\dfrac{5}{21}-\dfrac{1}{3}-\dfrac{2}{15}+\dfrac{2}{21}\)

\(=\left(\dfrac{-13}{15}-\dfrac{2}{15}\right)+\left(\dfrac{5}{21}+\dfrac{2}{21}-\dfrac{1}{3}\right)\)

\(=\dfrac{-15}{15}+\left(\dfrac{7}{21}-\dfrac{1}{3}\right)\)

\(=-1+\left(\dfrac{1}{3}-\dfrac{1}{3}\right)\)

\(=-1+0=-1\)

\(\dfrac{-13}{15}\) + \(\dfrac{5}{21}\) - \(\dfrac{1}{3}\) - \(\dfrac{2}{15}\) + \(\dfrac{2}{21}\)

=\(\dfrac{-13}{15}\) + \(\dfrac{2}{15}\) - \(\dfrac{5}{21}\) + \(\dfrac{2}{21}\) - \(\dfrac{1}{3}\)

= \(\dfrac{-15}{15}\) + \(\dfrac{7}{21}\) - \(\dfrac{1}{3}\)

= \(\dfrac{-3}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{3}\)

= \(\dfrac{-2}{3}\) - \(\dfrac{1}{3}\)

= \(\dfrac{-3}{3}\)

= -1

\(\dfrac{7}{4}.\dfrac{29}{5}-\dfrac{7}{5}.\dfrac{29}{4}+1\dfrac{1}{2}\)

\(=\dfrac{7}{5}.\dfrac{29}{4}-\dfrac{7}{5}.\dfrac{29}{4}+1\dfrac{1}{2}\)

\(=0+1\dfrac{1}{2}=1\dfrac{1}{2}=\dfrac{3}{2}\)

\(\dfrac{8}{21}-\dfrac{5}{11}-\left(-\dfrac{13}{21}\right)-\dfrac{6}{11}\)

\(=\dfrac{8}{21}-\dfrac{5}{11}+\dfrac{13}{21}-\dfrac{6}{11}\)

\(=\left(\dfrac{8}{21}+\dfrac{13}{21}\right)+\left(-\dfrac{5}{11}-\dfrac{6}{11}\right)\)

\(=1+\left(-1\right)=0\)

\(\dfrac{8}{21}-\dfrac{5}{11}-\left(-\dfrac{13}{21}\right)-\dfrac{6}{11}\)

\(=\dfrac{8}{21}-\dfrac{5}{11}+\dfrac{13}{21}-\dfrac{6}{11}\)

\(=\left(\dfrac{8}{21}+\dfrac{13}{21}\right)+\left(-\dfrac{5}{11}-\dfrac{6}{11}\right)\)

\(=\dfrac{21}{21}-\dfrac{11}{11}\)

\(=1-1=0\)