Math Olympiad for 3rd Grade – 1

Use symbol $+$, $-$, $\times$ and $\div$ to fill in the blanks to make the following equation: $$ \texttt{1}\ \boxed{　}\ \texttt{2}\ \boxed{　}\ \texttt{3}\ \boxed{　}\ \texttt{4}\ \boxed{　}\ \texttt{5}\ =\ \texttt{20}$$
Find the value of the following: $$9999999+999999+99999+9999+999+99+9$$
Find the value of the following: $$1+2-3+4-5+6-7+8-9+…+2018-2019+2020$$
Find the value of the following: $$1+2+3-4-5+6+7-8-9+…+2018+2019-2020$$
$a$,$b$,$c$,and $d$ are different single digit numbers and $$ \begin{array} \ \ \ \ \ \ \ \ \texttt{ a b c d}\\ \ \ \ \dfrac{\times　　　\texttt{　　　9}}{　\texttt{ d c b a }}\end{array}$$ The value of 4-digit number $abcd$ is __________.