C

7 túi có số kg gạo là:

\(2045:5\times7=2863\left(kg\right)\)

\(a,10;11;12;13;14;15;16;17;18;19\\ b,20;21;22;23;24;25;26;27;28;29\\ c,90;91;92;93;94;95;96;97;98;99\)

2 tiết kéo dài là:

\(8:50-7:30=1:20\)

Đổi 1:20 giờ = 80 phút

Mỗi tiết kéo dài là:

\(80:2=40\left(phút\right)\)

\(\dfrac{1}{2}-x=\dfrac{-2}{3}\\ x=\dfrac{1}{2}+\dfrac{2}{3}\\ x=\dfrac{7}{6}\)

\(4.x=4\dfrac{1}{3}\\ 4.x=\dfrac{13}{3}\\ x=\dfrac{13}{3}:4\\ x=\dfrac{13}{12}\)

\(4.x=4\dfrac{3}{4}\\ 4.x=\dfrac{19}{4}\\ x=\dfrac{19}{4}:4\\ x=\dfrac{19}{16}\)

\(\dfrac{-4}{8}-\left(\dfrac{-1}{3}-x\right)=\dfrac{2}{5}\\ \dfrac{-1}{3}-x=\dfrac{-4}{8}-\dfrac{2}{5}\\\dfrac{-1}{3}-x=\dfrac{-9}{10}\\ x=\dfrac{-1}{3}+\dfrac{9}{10}\\ x=\dfrac{17}{30} \)