$\begin{array}{l}1)\\a)\ A=\left(\dfrac{-2}3+1\dfrac14-\dfrac16\right)\cdot\dfrac{-24}{10}\\\to A=\left(\dfrac{-2}3+\dfrac54-\dfrac16\right)\cdot\dfrac{-12}5\\\to A=\left(\dfrac{-8}{12}+\dfrac{15}{12}-\dfrac{2}{12}\right)\cdot\dfrac{-12}5\\\to A=\dfrac5{12}\cdot\dfrac{-12}5\\\to A=-1\\\,\\b)\ B=\dfrac{13}{15}\cdot0,25\cdot3+\left(\dfrac8{15}-1\dfrac{19}{60}\right)\div1\dfrac{23}{24}\\\to B=\dfrac{13}{15}\cdot\dfrac14\cdot3+\left(\dfrac{32}{60}-\dfrac{79}{60}\right)\div\dfrac{47}{24}\\\to B=\dfrac{13.1.3}{15.4}+\dfrac{-47}{60}\cdot\dfrac{24}{47}\\\to B=\dfrac{13}{20}+\dfrac{-2}{5}\\\to B=\dfrac{13}{20}+\dfrac{-8}{20}\\\to B=\dfrac14\\\,\\2)\\a)\ 5,2.x+7\dfrac25=6\dfrac34\\\Leftrightarrow \dfrac{26x}5+\dfrac{37}5=\dfrac{27}4\\\Leftrightarrow \dfrac{26x+37}5=\dfrac{27}4\\\Leftrightarrow 4(26x+37)=5.27\\\Leftrightarrow 104x+148=135\\\Leftrightarrow 104x=-13\\\Leftrightarrow x=\dfrac{-1}{8}\\\,\\b)\ 2,4\div\left(\dfrac{-1}2-x\right)=1\dfrac35\\\Leftrightarrow \dfrac{12}5\div\left(\dfrac{-1}2-x\right)=\dfrac85\\\Leftrightarrow\dfrac{-1}2-x=\dfrac{12}5\div\dfrac85\\\Leftrightarrow \dfrac{-1}2-x=\dfrac32\\\Leftrightarrow x=\dfrac{-1}2-\dfrac32\\\Leftrightarrow x=-2\\\,\\3)\\\text{- Ta có : $40\%=\dfrac25$}\\\text{- Lượng nước vòi chảy giờ thứ 3 chiếm số phần bể là :}\\\quad\rm 1-\left(\dfrac25+\dfrac38\right)=\dfrac9{40}\ (bể)\\\text{- Dung tích bể là :}\\\quad 1080\div\dfrac9{40}=4800\ (l) \end{array}$
