\(x+\dfrac{21}{40}=\dfrac{53}{40}\)
\(x=\dfrac{4}{5}\)
\(x=\dfrac{53}{40}\times\dfrac{10}{7}-\dfrac{3}{4}=\dfrac{8}{7}\)
\(x+\dfrac{3}{4}\times\dfrac{7}{10}=\dfrac{53}{40}\)
\(\Leftrightarrow x+\dfrac{21}{40}=\dfrac{53}{40}\)
\(\Leftrightarrow x=\dfrac{53}{40}-\dfrac{21}{40}\)
\(\Leftrightarrow x=\dfrac{4}{5}\)
\(\)